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. Initial C++11 support for SWIG was written during the Google Summer of Code 2009 period.
SWIG supports all the new C++ syntax changes with some minor limitations (decltype expressions, variadic templates number). Wrappers for the new STL types (unordered_ containers, result_of, tuples) are not supported yet.
SWIG correctly parses the new operator && the same as the reference operator &.
The wrapper for the following code is correctly produced:
class MyClass {
MyClass(MyClass&& p) : ptr(p.ptr) {p.ptr = 0;}
MyClass& operator=(MyClass&& p) {
std::swap(ptr, p.ptr);
return *this;
}
};
SWIG correctly parses the keyword constexpr, but ignores its functionality. Constant functions cannot be used as constants.
constexpr int myConstFunc() { return 10; }
const int a = myConstFunc(); // results in error
Users needs to use values or predefined constants when defining the new constant value:
#define MY_CONST 10
constexpr int myConstFunc() { return MY_CONST; }
const int a = MY_CONST; // ok
SWIG correctly parses the keywords extern template. However, the explicit template instantiation is not used by SWIG, a %template is still required.
extern template class std::vector<MyClass>; // explicit instantiation
...
class MyClass {
public:
int a;
int b;
};
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 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.
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
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
This feature is part of the implementation block only. SWIG ignores it.
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.
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);
SWIG is able to handle constructor delegation, such as:
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; }
};
Constructor inheritance is parsed correctly, but the additional constructors are not currently added to the derived proxy class in the target language. Example is shown below:
class BaseClass {
public:
BaseClass(int iValue);
};
class DerivedClass: public BaseClass {
public:
using BaseClass::BaseClass; // Adds DerivedClass(int) constructor
};
The nullptr constant is largely unimportant in wrappers. In the few places it has an effect, it is treated like NULL.
SWIG parses the new enum class syntax and forward declarator for the enums:
enum class MyEnum : unsigned int;
The strongly typed enumerations are treated the same as the ordinary and anonymous enums. This is because SWIG doesn't support nested classes. This is usually not a problem, however, there may be some name clashes. For example, the following code:
class Color {
enum class PrintingColors : unsigned int {
Cyan, Magenta, Yellow, Black
};
enum class BasicColors {
Red, Green, Blue
};
enum class AllColors {
// produces warnings because of duplicate names
Yellow, Orange, Red, Magenta, Blue, Cyan, Green, Pink, Black, White
};
};
A workaround is to write these as a series of separated classes containing anonymous enums:
class PrintingColors {
enum : unsigned int {
Cyan, Magenta, Yellow, Black
};
};
class BasicColors {
enum : unsigned int {
Red, Green, Blue
};
};
class AllColors {
enum : unsigned int {
Yellow, Orange, Red, Magenta, Blue, Cyan, Green, Pink, Black, White
};
};
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;
SWIG correctly parses the keyword explicit both for operators and constructors. 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;
};
The usage of explicit constructors and operators is somehow specific to C++ when assigning the value of one object to another one of different type or translating one type to another. It requires both operator and function overloading features, which are not supported by the majority of SWIG target languages. Also the constructors and operators are not particulary useful in any SWIG target languages, because all use their own facilities (eg. classes Cloneable and Comparable in Java) to achieve particular copy and compare behaviours.
The following is an example of an alias template:
template< typename T1, typename T2, int >
class SomeType {
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
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 // 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;
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.
SWIG supports 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";
Note: SWIG currently incorrectly parses the odd number of double quotes inside the string due to SWIG's C++ preprocessor.
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;
SWIG correctly parses the thread_local keyword. For example, variable 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.
SWIG correctly parses the = delete and = default keywords. For example:
struct NonCopyable {
NonCopyable& operator=(const NonCopyable&) = delete; /* Removes operator= */
NonCopyable(const NonCopyable&) = delete; /* Removed copy constructor */
NonCopyable() = default; /* Explicitly allows the empty constructor */
void *operator new(std::size_t) = delete; /* Removes new NonCopyable */
};
This feature is specific to C++ only. The defaulting/deleting is currently ignored, because SWIG automatically produces wrappers for special constructors and operators specific to the target language.
SWIG correctly parses and uses the new long long type already introduced in C99 some time ago.
SWIG correctly parses and calls the new static_assert function.
template <typename T>
struct Check {
static_assert(sizeof(int) <= sizeof(T), "not big enough");
};
SWIG correctly calls the sizeof() on types as well as on the 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
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 that do not rely on C++.
SWIG does not wrap the new tuple types and the unordered_ container classes yet. Variadic template support is working so it is possible to include the tuple header file; it is parsed without any problems.
SWIG does not wrap the new C++11 regular expressions classes, because the SWIG target languages use their own facilities for this.
SWIG provides special smart pointer handling for std::tr1::shared_ptr in the same way it has support for boost::shared_ptr. There is no special smart pointer handling available for std::weak_ptr and std::unique_ptr.
This feature extends and standardizes the standard library only and does not effect the C++ language and SWIG.
The new ref and cref classes are used to instantiate a parameter as a reference of a template function. For example:
void f( int &r ) { r++; }
// Template function.
template< class F, class P > void g( F f, P t ) { f(t); }
int main() {
int i = 0 ;
g( f, i ) ; // 'g<void ( int &r ), int>' is instantiated
// then 'i' will not be modified.
cout << i << endl ; // Output -> 0
g( f, ref(i) ) ; // 'g<void(int &r),reference_wrapper<int>>' is instantiated
// then 'i' will be modified.
cout << i << endl ; // Output -> 1
}
The ref and cref classes are not wrapped by SWIG because the SWIG target languages do not support referencing.
SWIG supports functor classes in some 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.
The new C++ metaprogramming is useful at compile time and is aimed specifically for C++ development:
// First way of operating.
template< bool B > struct algorithm {
template< class T1, class T2 > int do_it( T1&, T2& ) { /*...*/ }
};
// Second way of operating.
template<> struct algorithm<true> {
template< class T1, class T2 > int do_it( T1, T2 ) { /*...*/ }
};
// Instantiating 'elaborate' will automatically instantiate the correct way to operate.
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
// in floating point, otherwise use the first way.
return algorithm< is_integral<T1>::value && is_floating_point<T2>::value >::do_it( A, B );
}
SWIG correctly parses the template specialization, template types and values inside the <> block and the new helper functions: is_convertible, is_integral, is_const etc. However, SWIG still explicitly requires concrete types when using the %template directive, so the C++ metaprogramming features are not really of interest at runtime in the target languages.
SWIG does not wrap the new result_of class introduced in the <functional> header and map the result_of::type to the concrete type yet. For example:
%inline %{
#include <functional>
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<double(*)(double), double>;
%constant double (*SQUARE)(double) = square;
will result in:
>>> test_result_impl(SQUARE, 5.0) <SWIG Object of type 'std::result_of< Fun(Arg) >::type *' at 0x7faf99ed8a50>
Instead, please use decltype() where possible for now.