# 7 SWIG and C++11

- Introduction
- Core language changes
  - Rvalue reference and move semantics
  - Generalized constant expressions
  - Extern template
  - Initializer lists
  - Uniform initialization
  - Type inference
  - Range-based for-loop
  - Lambda functions and expressions
  - Alternate function syntax
  - Object construction improvement
  - Explicit overrides and final
  - Null pointer constant
  - Strongly typed enumerations
  - Double angle brackets
  - Explicit conversion operators
  - Alias templates
  - Unrestricted unions
  - Variadic templates
  - New string literals
  - User-defined literals
  - Thread-local storage
  - Explicitly defaulted functions and deleted functions
  - Type long long int
  - Static assertions
  - Allow sizeof to work on members of classes without an explicit object
  - Exception specifications and noexcept
  - Control and query object alignment
  - Attributes
- Standard library changes
  - Threading facilities
  - Tuple types
  - Hash tables
  - Regular expressions
  - General-purpose smart pointers
  - Extensible random number facility
  - Wrapper reference
  - Polymorphous wrappers for function objects
  - Type traits for metaprogramming
  - Uniform method for computing return type of function objects

## 7.1 Introduction

This chapter gives you a brief overview about the SWIG implementation of the C++11 standard. This part of SWIG is still a work in progress.

SWIG supports the new C++ syntax changes with some minor limitations in some areas such as decltype expressions and variadic templates. Wrappers for the new STL types (unordered_ containers, result_of, tuples) are incomplete. The wrappers for the new containers would work much like the C++03 containers and users are welcome to help by adapting the existing container interface files and submitting them as a patch for inclusion in future versions of SWIG.

## 7.2 Core language changes

### 7.2.1 Rvalue reference and move semantics

SWIG correctly parses the rvalue reference syntax '&&', for example the typical usage of it in the move constructor and move assignment operator below:

    class MyClass {
    ...
      std::vector<int> numbers;
    public:
      MyClass(MyClass &&other) : numbers(std::move(other.numbers)) {}
      MyClass & operator=(MyClass &&other) {
        numbers = std::move(other.numbers);
        return *this;
      }
    };

Rvalue references are designed for C++ temporaries and so are not very useful when used from non-C++ target languages. Generally you would just ignore them via `%ignore` before parsing the class. For example, ignore the move constructor:

    %ignore MyClass::MyClass(MyClass &&);

The plan is to ignore move constructors by default in a future version of SWIG. Note that both normal assignment operators as well as move assignment operators are ignored by default in most target languages with the following warning:

    example.i:18: Warning 503: Can't wrap 'operator =' unless renamed to a valid identifier.

### 7.2.2 Generalized constant expressions

SWIG parses and identifies the keyword `constexpr`, but cannot fully utilise it. These C++ compile time constants are usable as runtime constants from the target languages. Below shows example usage for assigning a C++ compile time constant from a compile time constant function:

    constexpr int XXX() { return 10; }
    constexpr int YYY = XXX() + 100;

When either of these is used from a target language, a runtime call is made to obtain the underlying constant.

### 7.2.3 Extern template

SWIG correctly parses the keywords `extern template`. However, this template instantiation suppression in a translation unit has no relevance outside of the C++ compiler and so is not used by SWIG. SWIG only uses `%template` for instantiating and wrapping templates.

    template class std::vector<int>;        // C++03 explicit instantiation in C++
    extern template class std::vector<int>; // C++11 explicit instantiation suppression in C++
    %template(VectorInt) std::vector<int>;  // SWIG instantiation

### 7.2.4 Initializer lists

Initializer lists are very much a C++ compiler construct and are not very accessible from wrappers as they are intended for compile time initialization of classes using the special `std::initializer_list` type. SWIG detects usage of initializer lists and will emit a special informative warning each time one is used:

    example.i:33: Warning 476: Initialization using std::initializer_list.

Initializer lists usually appear in constructors but can appear in any function or method. They often appear in constructors which are overloaded with alternative approaches to initializing a class, such as the std container's push_back method for adding elements to a container. The recommended approach then is to simply ignore the initializer-list constructor, for example:

    %ignore Container::Container(std::initializer_list<int>);
    class Container {
    public:
      Container(std::initializer_list<int>); // initializer-list constructor
      Container();
      void push_back(const int &);
      ...
    };

Alternatively you could modify the class and add another constructor for initialization by some other means, for example by a `std::vector`:

    %include <std_vector.i>
    class Container {
    public:
      Container(const std::vector<int> &);
      Container(std::initializer_list<int>); // initializer-list constructor
      Container();
      void push_back(const int &);
      ...
    };

And then call this constructor from your target language, for example, in Python, the following will call the constructor taking the `std::vector`:

    >>> c = Container( [1,2,3,4] )

If you are unable to modify the class being wrapped, consider ignoring the initializer-list constructor and using %extend to add in an alternative constructor:

    %include <std_vector.i>
    %extend Container {
      Container(const std::vector<int> &elements) {
        Container *c = new Container();
        for (int element : elements)
          c->push_back(element);
        return c;
      }
    }

    %ignore Container::Container(std::initializer_list<int>);

    class Container {
    public:
      Container(std::initializer_list<int>); // initializer-list constructor
      Container();
      void push_back(const int &);
      ...
    };

The above makes the wrappers look is as if the class had been declared as follows:

    %include <std_vector.i>
    class Container {
    public:
      Container(const std::vector<int> &);
    //  Container(std::initializer_list<int>); // initializer-list constructor (ignored)
      Container();
      void push_back(const int &);
      ...
    };

`std::initializer_list` is simply a container that can only be initialized at compile time. As it is just a C++ type, it is possible to write typemaps for a target language container to map onto `std::initializer_list`. However, this can only be done for a fixed number of elements as initializer lists are not designed to be constructed with a variable number of arguments at runtime. The example below is a very simple approach which ignores any parameters passed in and merely initializes with a fixed list of fixed integer values chosen at compile time:

    %typemap(in) std::initializer_list<int> {
      $1 = {10, 20, 30, 40, 50};
    }
    class Container {
    public:
      Container(std::initializer_list<int>); // initializer-list constructor
      Container();
      void push_back(const int &);
      ...
    };

Any attempt at passing in values from the target language will be ignored and be replaced by `{10, 20, 30, 40, 50}`. Needless to say, this approach is very limited, but could be improved upon, but only slightly. A typemap could be written to map a fixed number of elements on to the `std::initializer_list`, but with values decided at runtime. The typemaps would be target language specific.

Note that the default typemap for `std::initializer_list` does nothing but issue the warning and hence any user supplied typemaps will override it and suppress the warning.

### 7.2.5 Uniform initialization

The curly brackets {} for member initialization are fully supported by SWIG:

    struct BasicStruct {
     int x;
     double y;
    };

    struct AltStruct {
      AltStruct(int x, double y) : x_{x}, y_{y} {}

      int x_;
      double y_;
    };

    BasicStruct var1{5, 3.2}; // only fills the struct components
    AltStruct var2{2, 4.3};   // calls the constructor

Uniform initialization does not affect usage from the target language, for example in Python:

    >>> a = AltStruct(10, 142.15)
    >>> a.x_
    10
    >>> a.y_
    142.15

### 7.2.6 Type inference

SWIG supports `decltype()` with some limitations. Single variables are allowed, however, expressions are not supported yet. For example, the following code will work:

    int i;
    decltype(i) j;

However, using an expression inside the decltype results in syntax error:

    int i; int j;
    decltype(i+j) k;  // syntax error

### 7.2.7 Range-based for-loop

This feature is part of the implementation block only. SWIG ignores it.

### 7.2.8 Lambda functions and expressions

SWIG correctly parses most of the Lambda functions syntax. For example:

    auto val = [] { return something; };
    auto sum = [](int x, int y) { return x+y; };
    auto sum = [](int x, int y) -> int { return x+y; };

The lambda functions are removed from the wrappers for now, because of the lack of support for closures (scope of the lambda functions) in the target languages.

Lambda functions used to create variables can also be parsed, but due to limited support of `auto` when the type is deduced from the expression, the variables are simply ignored.

    auto six = [](int x, int y) { return x+y; }(4, 2);

Better support should be available in a later release.

### 7.2.9 Alternate function syntax

SWIG fully supports the new definition of functions. For example:

    struct SomeStruct {
      int FuncName(int x, int y);
    };

can now be written as in C++11:

    struct SomeStruct {
      auto FuncName(int x, int y) -> int;
    };

    auto SomeStruct::FuncName(int x, int y) -> int {
      return x + y;
    }

The usage in the target languages remains the same, for example in Python:

    >>> a = SomeStruct()
    >>> a.FuncName(10,5)
    15

SWIG will also deal with type inference for the return type, as per the limitations described earlier. For example:

    auto square(float a, float b) -> decltype(a);

### 7.2.10 Object construction improvement

There are three parts to object construction improvement. The first improvement is constructor delegation such as the following:

    class A {
    public:
      int a;
      int b;
      int c;

      A() : A(10) {}
      A(int aa) : A(aa, 20) {}
      A(int aa, int bb) : A(aa, bb, 30) {}
      A(int aa, int bb, int cc) { a=aa; b=bb; c=cc; }
    };

where peer constructors can be called. SWIG handles this without any issue.

The second improvement is constructor inheritance via a `using` declaration. This is parsed correctly, but the additional constructors are not currently added to the derived proxy class in the target language. An example is shown below:

    class BaseClass {
    public:
      BaseClass(int iValue);
    };

    class DerivedClass: public BaseClass {
      public:
      using BaseClass::BaseClass; // Adds DerivedClass(int) constructor
    };

The final part is member initialization at the site of the declaration. This kind of initialization is handled by SWIG.

    class SomeClass {
    public:
        SomeClass() {}
        explicit SomeClass(int new_value) : value(new_value) {}

        int value = 5;
    };

### 7.2.11 Explicit overrides and final

The special identifiers `final` and `override` can be used on methods and destructors, such as in the following example:

    struct BaseStruct {
      virtual void ab() const = 0;
      virtual void cd();
      virtual void ef();
      virtual ~BaseStruct();
    };
    struct DerivedStruct : BaseStruct {
      virtual void ab() const override;
      virtual void cd() final;
      virtual void ef() final override;
      virtual ~DerivedStruct() override;
    };

### 7.2.12 Null pointer constant

The `nullptr` constant is mostly unimportant in wrappers. In the few places it has an effect, it is treated like `NULL`.

### 7.2.13 Strongly typed enumerations

SWIG supports strongly typed enumerations and parses the new `enum class` syntax and forward declarator for the enums, such as:

    enum class MyEnum : unsigned int;

Strongly typed enums are often used to avoid name clashes such as the following:

    struct Color {
      enum class RainbowColors : unsigned int {
        Red, Orange, Yellow, Green, Blue, Indigo, Violet
      };

      enum class WarmColors {
        Yellow, Orange, Red
      };

      // Note normal enum
      enum PrimeColors {
        Red=100, Green, Blue
      };
    };

There are various ways that the target languages handle enums, so it is not possible to precisely state how they are handled in this section. However, generally, most scripting languages mangle in the strongly typed enumeration's class name, but do not use any additional mangling for normal enumerations. For example, in Python, the following code

    print Color.RainbowColors_Red, Color.WarmColors_Red, Color.Red

results in

    0 2 100

The strongly typed languages often wrap normal enums into an enum class and so treat normal enums and strongly typed enums the same. The equivalent in Java is:

    System.out.println(Color.RainbowColors.Red.swigValue() + " " + Color.WarmColors.Red.swigValue() + " " + Color.PrimeColors.Red.swigValue());

### 7.2.14 Double angle brackets

SWIG correctly parses the symbols >> as closing the template block, if found inside it at the top level, or as the right shift operator >> otherwise.

    std::vector<std::vector<int>> myIntTable;

### 7.2.15 Explicit conversion operators

SWIG correctly parses the keyword `explicit` for operators in addition to constructors now. For example:

    class U {
    public:
      int u;
    };

    class V {
    public:
      int v;
    };

    class TestClass {
    public:
      //implicit converting constructor
      TestClass(U const &val) { t=val.u; }

      // explicit constructor
      explicit TestClass(V const &val) { t=val.v; }

      int t;
    };

    struct Testable {
      // explicit conversion operator
      explicit operator bool() const {
        return false;
      }
    };

The effect of explicit constructors and operators has little relevance for the proxy classes as target languages don't have the same concepts of implicit conversions as C++. Conversion operators either with or without `explicit` need renaming to a valid identifier name in order to make them available as a normal proxy method.

### 7.2.16 Alias templates

The following is an example of an alias template:

    template< typename T1, typename T2, int >
    class SomeType {
    public:
      T1 a;
      T2 b;
      int c;
    };

    template< typename T2 >
    using TypedefName = SomeType<char*, T2, 5>;

These are partially supported as SWIG will parse these and identify them, however, they are ignored as they are not added to the type system. A warning such as the following is issued:

    example.i:13: Warning 342: The 'using' keyword in template aliasing is not fully supported yet.

Similarly for non-template type aliasing:

    using PFD = void (*)(double); // New introduced syntax

A warning will be issued:

    example.i:17: Warning 341: The 'using' keyword in type aliasing is not fully supported yet.

The equivalent old style typedefs can be used as a workaround:

    typedef void (*PFD)(double);  // The old style

### 7.2.17 Unrestricted unions

SWIG fully supports any type inside a union even if it does not define a trivial constructor. For example, the wrapper for the following code correctly provides access to all members in the union:

    struct point {
      point() {}
      point(int x, int y) : x_(x), y_(y) {}
      int x_, y_;
    };

    #include <new> // For placement 'new' in the constructor below
    union P {
      int z;
      double w;
      point p; // Illegal in C++03; legal in C++11.
      // Due to the point member, a constructor definition is required.
      P() {
        new(&p) point();
      }
    } p1;

### 7.2.18 Variadic templates

SWIG supports the variadic templates syntax (inside the <> block, variadic class inheritance and variadic constructor and initializers) with some limitations. The following code is correctly parsed:

    template <typename... BaseClasses> class ClassName : public BaseClasses... {
    public:
       ClassName (BaseClasses &&... baseClasses) : BaseClasses(baseClasses)... {}
    }

For now however, the `%template` directive only accepts one parameter substitution for the variable template parameters.

    %template(MyVariant1) ClassName<>         // zero argument not supported yet
    %template(MyVariant2) ClassName<int>      // ok
    %template(MyVariant3) ClassName<int, int> // too many arguments not supported yet

Support for the variadic `sizeof()` function is correctly parsed:

    const int SIZE = sizeof...(ClassName<int, int>);

In the above example `SIZE` is of course wrapped as a constant.

### 7.2.19 New string literals

SWIG supports wide string and Unicode string constants and raw string literals.

    // New string literals
    wstring         aa =  L"Wide string";
    const char     *bb = u8"UTF-8 string";
    const char16_t *cc =  u"UTF-16 string";
    const char32_t *dd =  U"UTF-32 string";

    // Raw string literals
    const char      *xx =        ")I'm an \"ascii\" \\ string.";
    const char      *ee =   R"XXX()I'm an "ascii" \ string.)XXX"; // same as xx
    wstring          ff =  LR"XXX(I'm a "raw wide" \ string.)XXX";
    const char      *gg = u8R"XXX(I'm a "raw UTF-8" \ string.)XXX";
    const char16_t  *hh =  uR"XXX(I'm a "raw UTF-16" \ string.)XXX";
    const char32_t  *ii =  UR"XXX(I'm a "raw UTF-32" \ string.)XXX";

Non-ASCII string support varies quite a bit among the various target languages though.

Note: There is a bug currently where SWIG's preprocessor incorrectly parses an odd number of double quotes inside raw string literals.

### 7.2.20 User-defined literals

SWIG parses the declaration of user-defined literals, that is, the `operator "" _mysuffix()` function syntax.

Some examples are the raw literal:

    OutputType operator "" _myRawLiteral(const char * value);

numeric cooked literals:

    OutputType operator "" _mySuffixIntegral(unsigned long long);
    OutputType operator "" _mySuffixFloat(long double);

and cooked string literals:

    OutputType operator "" _mySuffix(const char * string_values, size_t num_chars);
    OutputType operator "" _mySuffix(const wchar_t * string_values, size_t num_chars);
    OutputType operator "" _mySuffix(const char16_t * string_values, size_t num_chars);
    OutputType operator "" _mySuffix(const char32_t * string_values, size_t num_chars);

Like other operators that SWIG parses, a warning is given about renaming the operator in order for it to be wrapped:

    example.i:27: Warning 503: Can't wrap 'operator "" _myRawLiteral' unless renamed to a valid identifier.

If %rename is used, then it can be called like any other wrapped method. Currently you need to specify the full declaration including parameters for %rename:

    %rename(MyRawLiteral)  operator"" _myRawLiteral(const char * value);

Or if you just wish to ignore it altogether:

    %ignore operator "" _myRawLiteral(const char * value);

Note that use of user-defined literals such as the following still give a syntax error:

    OutputType var1 = "1234"_suffix;
    OutputType var2 = 1234_suffix;
    OutputType var3 = 3.1416_suffix;

### 7.2.21 Thread-local storage

SWIG correctly parses the `thread_local` keyword. For example, variables reachable by the current thread can be defined as:

    struct A {
       static thread_local int val;
    };
    thread_local int global_val;

The use of the `thread_local` storage specifier does not affect the wrapping process; it does not modify the wrapper code compared to when it is not specified. A variable will be thread local if accessed from different threads from the target language in the same way that it will be thread local if accessed from C++ code.

### 7.2.22 Explicitly defaulted functions and deleted functions

SWIG handles explicitly defaulted functions, that is, `= default` added to a function declaration. Deleted definitions, which are also called deleted functions, have `= delete` added to the function declaration. For example:

    struct NonCopyable {
      NonCopyable & operator=(const NonCopyable &) = delete; /* Removes operator= */
      NonCopyable(const NonCopyable &) = delete;             /* Removes copy constructor */
      NonCopyable() = default;                               /* Explicitly allows the empty constructor */
    };

Wrappers for deleted functions will not be available in the target language. Wrappers for defaulted functions will of course be available in the target language. Explicitly defaulted functions have no direct effect for SWIG wrapping as the declaration is handled much like any other method declaration parsed by SWIG.

Deleted functions are also designed to prevent implicit conversions when calling the function. For example, the C++ compiler will not compile any code which attempts to use an int as the type of the parameter passed to `f` below:

    struct NoInt {
        void f(double i);
        void f(int) = delete;
    };

This is a C++ compile time check and SWIG does not make any attempt to detect if the target language is using an int instead of a double though, so in this case it is entirely possible to pass an int instead of a double to `f` from Java, Python etc.

### 7.2.23 Type long long int

SWIG correctly parses and uses the new `long long` type already introduced in C99 some time ago.

### 7.2.24 Static assertions

SWIG correctly parses the new `static_assert` declarations. This is a C++ compile time directive so there isn't anything useful that SWIG can do with it.

    template <typename T>
    struct Check {
      static_assert(sizeof(int) <= sizeof(T), "not big enough");
    };

### 7.2.25 Allow sizeof to work on members of classes without an explicit object

SWIG can parse the new sizeof() on types as well as on objects. For example:

    struct A {
      int member;
    };

    const int SIZE = sizeof(A::member); // does not work with C++03. Okay with C++11

In Python:

    >>> SIZE
    8

### 7.2.26 Exception specifications and noexcept

C++11 added in the noexcept specification to exception specifications to indicate that a function simply may or may not throw an exception, without actually naming any exception. SWIG understands these, although there isn't any useful way that this information can be taken advantage of by target languages, so it is as good as ignored during the wrapping process. Below are some examples of noexcept in function declarations:

    static void noex1() noexcept;
    int noex2(int) noexcept(true);
    int noex3(int, bool) noexcept(false);

### 7.2.27 Control and query object alignment

An `alignof` operator is used mostly within C++ to return alignment in number of bytes, but could be used to initialize a variable as shown below. The variable's value will be available for access by the target language as any other variable's compile time initialised value.

    const int align1 = alignof(A::member);

The `alignas` specifier for variable alignment is not yet supported. Example usage:

    struct alignas(16) S {
      int num;
    };
    alignas(double) unsigned char c[sizeof(double)];

Use the preprocessor to work around this for now:

    #define alignas(T)

### 7.2.28 Attributes

Attributes such as those shown below, are not yet supported and will give a syntax error.

    int [[attr1]] i [[attr2, attr3]];

    [[noreturn, nothrow]] void f [[noreturn]] ();

## 7.3 Standard library changes

### 7.3.1 Threading facilities

SWIG does not currently wrap or use any of the new threading classes introduced (thread, mutex, locks, condition variables, task). The main reason is that SWIG target languages offer their own threading facilities so there is limited use for them.

### 7.3.2 Tuple types

SWIG does not provide library files for the new tuple types yet. Variadic template support requires further work to provide substantial tuple wrappers.

### 7.3.3 Hash tables

The new hash tables in the STL are `unordered_set`, `unordered_multiset`, `unordered_map`, `unordered_multimap`. These are not available in SWIG, but in principle should be easily implemented by adapting the current STL containers.

### 7.3.4 Regular expressions

While SWIG could provide wrappers for the new C++11 regular expressions classes, there is little need as the target languages have their own regular expression facilities.

### 7.3.5 General-purpose smart pointers

SWIG provides special smart pointer handling for `std::shared_ptr` in the same way it has support for `boost::shared_ptr`. Please see the shared_ptr smart pointer library section. There is no special smart pointer handling available for `std::weak_ptr` and `std::unique_ptr` yet.

### 7.3.6 Extensible random number facility

This feature extends and standardizes the standard library only and does not effect the C++ language nor SWIG.

### 7.3.7 Wrapper reference

Wrapper references are similar to normal C++ references but are copy-constructible and copy-assignable. They could conceivably be used in public APIs. There is no special support for `std::reference_wrapper` in SWIG though. Users would need to write their own typemaps if wrapper references are being used and these would be similar to the plain C++ reference typemaps.

### 7.3.8 Polymorphous wrappers for function objects

SWIG supports functor classes in a few languages in a very natural way. However nothing is provided yet for the new `std::function` template. SWIG will parse usage of the template like any other template.

    %rename(__call__) Test::operator(); // Default renaming used for Python

    struct Test {
      bool operator()(int x, int y); // function object
    };

    #include <functional>
    std::function<void (int, int)> pF = Test;   // function template wrapper

Example of supported usage of the plain functor from Python is shown below. It does not involve `std::function`.

    t = Test()
    b = t(1,2) # invoke C++ function object

### 7.3.9 Type traits for metaprogramming

The type_traits functions to support C++ metaprogramming is useful at compile time and is aimed specifically at C++ development:

    #include <type_traits>

    // First way of operating.
    template< bool B > struct algorithm {
      template< class T1, class T2 > static int do_it(T1 &, T2 &)  { /*...*/ return 1; }
    };

    // Second way of operating.
    template<> struct algorithm<true> {
      template< class T1, class T2 > static int do_it(T1, T2)  { /*...*/ return 2; }
    };

    // Instantiating 'elaborate' will automatically instantiate the correct way to operate, depending on the types used.
    template< class T1, class T2 > int elaborate(T1 A, T2 B) {
      // Use the second way only if 'T1' is an integer and if 'T2' is a floating point,
      // otherwise use the first way.
      return algorithm< std::is_integral<T1>::value && std::is_floating_point<T2>::value >::do_it(A, B);
    }

SWIG correctly parses the template specialization, template types etc. However, metaprogramming and the additional support in the type_traits header is really for compile time and is not much use at runtime for the target languages. For example, as SWIG requires explicit instantiation of templates via `%template`, there isn't much that `std::is_integral<int>` is going to provide by itself. However, template functions using such metaprogramming techniques might be useful to wrap. For example, the following instantiations could be made:

    %template(Elaborate) elaborate<int, int>;
    %template(Elaborate) elaborate<int, double>;

Then the appropriate algorithm can be called for the subset of types given by the above `%template` instantiations from a target language, such as Python:

    >>> Elaborate(0, 0)
    1
    >>> Elaborate(0, 0.0)
    2

### 7.3.10 Uniform method for computing return type of function objects

The new `std::result_of` class introduced in the <functional> header provides a generic way to obtain the return type of a function type via `std::result_of::type`. There isn't any library interface file to support this type. With a bit of work, SWIG will deduce the return type of functions when used in `std::result_of` using the approach shown below. The technique basically forward declares the `std::result_of` template class, then partially specializes it for the function types of interest. SWIG will use the partial specialization and hence correctly use the `std::result_of::type` provided in the partial specialization.

    %inline %{
    #include <functional>
    typedef double(*fn_ptr)(double);
    %}

    namespace std {
      // Forward declaration of result_of
      template<typename Func> struct result_of;
      // Add in a partial specialization of result_of
      template<> struct result_of< fn_ptr(double) > {
        typedef double type;
      };
    }

    %template() std::result_of< fn_ptr(double) >;

    %inline %{

    double square(double x) {
      return (x * x);
    }

    template<class Fun, class Arg>
    typename std::result_of<Fun(Arg)>::type test_result_impl(Fun fun, Arg arg) {
      return fun(arg);
    }
    %}

    %template(test_result) test_result_impl< fn_ptr, double >;
    %constant double (*SQUARE)(double) = square;

Note the first use of `%template` which SWIG requires to instantiate the template. The empty template instantiation suffices as no proxy class is required for `std::result_of<Fun(Arg)>::type` as this type is really just a `double`. The second `%template` instantiates the template function which is being wrapped for use as a callback. The `%constant` can then be used for any callback function as described in Pointers to functions and callbacks.

Example usage from Python should give the not too surprising result:

    >>> test_result(SQUARE, 5.0)
    25.0

Phew, that is a lot of hard work to get a callback working. You could just go with the more attractive option of just using `double` as the return type in the function declaration instead of `result_of`!