c++11 metaprogramming: check for method existence [duplicate] - c++

Question

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Templated check for the existence of a class member function?
(33 answers)
Closed 6 years ago.
1) I have two classes class A and class B which both have a method called foo but with different argument lists.
class A {
public:
void foo(int a);
};
class B {
public:
void foo(int a, int b);
};
2) Further, I have a class C with template argument T that also has a foo method as following
template <typename T>
class C {
public:
void foo();
private:
T t_;
int a_;
int b_;
};
3) I would like use both class A and class B as template argument for class C.
Saying that I would like to have a method C::foo to be implemented like following:
template <typename T>
void C<T>::foo()
{
if (compile time check: T has foo(int a, int b))
t_.foo(a_, b_);
else
t_.foo(a_);
}
How can I express the if statement above in C++11?

Answer 1

Use SFINAE (with function template overloading).
template <typename T>
class C {
private:
T t_;
int a_;
int b_;
public:
template <typename X = T>
auto foo() -> decltype (std::declval<X>().foo(a_)) {
t_.foo(a_);
}
template <typename X = T>
auto foo() -> decltype (std::declval<X>().foo(a_, b_)) {
t_.foo(a_, b_);
}
};
LIVE

Answer 2

If you know which type contains void foo(int a, int b); which in this case you do, you can use template specialization, something like
template <typename T>
class C
{
private:
T t_;
int a_;
int b_;
public:
// called for general use
void foo()
{
t_.foo(a_);
}
};
// specialized version for T = B
template<>
void C<B>::foo()
{
t_.foo(a_, b_);
}