how to decide by 'int' type of 'vector<int>' in C++?

Question

I can use enable_if to separate behavior by parameter type such as:

std::vector<int>

Now I want to separate behavior by the inner type of a container:

int of std::vector<int>

what can I do in c++?

Answer 1

I wonder if this is what you mean:

#include<iostream>
#include<vector>
#include<type_traits>

// The following function will display the contents of the provided T
// only if its value_type trait is of type int.
template<typename T>
typename std::enable_if<
  std::is_same<typename T::value_type, int>::value,
  void>::type display(const T& data) {
  std::cout<<"Some ints:"<<std::endl;
  for(int xi : data) {
    std::cout<<xi<<" ";
  }
  std::cout<<std::endl;
}

int main() {
  std::vector<int> dint = {1, 2, 3, 4, 5, 6};
  display(dint);

  std::vector<float> dfloat = {1, 2, 3, 4, 5, 6};

  // The following call would lead to compile-time error, because
  // there is no display function activated for float types:

  //display(dfloat);
  return 0;
}

Compiling with g++ example.cpp -std=c++11 -Wall -Wextra (OS X 10.7.4 using GCC 4.8.1) yields:

$ ./a.out 
Some ints:
1 2 3 4 5 6 

As expected, if I uncomment the display(dfloat) line the compiler error message includes:

error: no matching function for call to ‘display(std::vector<float>&)’

Answer 2

Something like the following, where you can fill in int, double, etc. for S?

std::vector<std::enable_if<std::is_same<T, S>::value, S>::type>