how to get template parameter of a template template parameter? - c++

Question

Assume std::vector didnt have a value_type. Is is possible to write a template to deduce the value_type? Or more general, given a T<X>, how can I deduce X?
A very naive..
template <template <typename X> T>
void test(T<X> t) {
X x;
}
will probably make anybody who knows a bit about templates laugh at my foolish attempt and when instantiated like this:
int main() {
std::vector<int> x;
test(x);
}
create the following errors:
error: expected ‘class’ before ‘T’
template < template<typename X> T>
^
error: ‘X’ was not declared in this scope
void test(T<X> u) {
^
error: template argument 1 is invalid
void test(T<X> u) {
^
In function ‘void test(int)’:
error: ‘X’ was not declared in this scope
X x;
^
error: expected ‘;’ before ‘x’
X x;
^
In function ‘int main()’:
error: no matching function for call to ‘test(std::vector<int>&)’
test(x);
^
note: candidate is:
note: template<template<class X> class T> void test(int)
void test(T<X> u) {
^
note: template argument deduction/substitution failed:
note: cannot convert ‘x’ (type ‘std::vector<int>’) to type ‘int’
EDIT: the first one is easy to fix, but fixing it wont affect the others...
PS: I think I have a small misunderstanding, as std::vector<int> isnt a template, but a concrete type. However, still I would like to know if there is a way to get the int from a someTemplate<int> with some template magic.

Answer 1

You can create a traits to extract those parameter:
template <typename T> struct first_param;
template <template <typename, typename...> class C, typename T, typename ...Ts>
struct first_param<C<T, Ts...>>
{
using type = T;
};
For pre C++11, you have to handle number of parameters until acceptable values:
template <typename T> struct first_param;
template <template <typename> class C, typename T>
struct first_param<C<T>>
{
typedef T type;
};
template <template <typename, typename> class C, typename T, typename T2>
struct first_param<C<T, T2>>
{
typedef T type;
};
template <template <typename, typename, typename> class C,
typename T, typename T2, typename T3>
struct first_param<C<T, T2, T3>>
{
typedef T type;
};
// ...