template template arguments: no matching call found

Question

#include <vector>

using std::vector;

template<template<typename> class x>
void test() {
    x<int> a;
    a.push_back(1);
}

int main() {
    test<vector>();
    return 0;
}

For some reason, I get no matching call error, despite my expectations. Why does this happen?

Where is the type `x` defined? Why are you `return`ing from a `void` function? — 3Dave, Oct 05 '16 at 13:03
See http://stackoverflow.com/questions/213761/what-are-some-uses-of-template-template-parameters-in-c for some syntax inspiration. — 3Dave, Oct 05 '16 at 13:05

Holt · Accepted Answer · 2016-10-05T13:05:27.057

4

std::vector does not take one argument, but two (there is Allocator), so it cannot be matched against a template that should take only one. You need to change to either:

template <template <typename, typename> class x>
// or:
template <template <typename... > class x>

You also need to change the return type of your function because it is unlikely that x<int> is void.

Note that if you use the version with two typename, you would need to specify each parameters in the return statement (e.g. x<int, std::allocator<int>>), which is why you should prefer the variadic version (typename...).

edited Oct 05 '16 at 13:05

answered Oct 05 '16 at 13:04

Holt

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So I should list each and every template parameter? – RomaValcer Oct 05 '16 at 13:05
1

@RomaValcer Yes, or use a variadic version. – Holt Oct 05 '16 at 13:05

score 2 · Answer 2 · answered Oct 05 '16 at 13:07

std::vector template looks as follows:

template<
    class T,
    class Allocator = std::allocator<T>
> class vector;

so you have typename T and allocator for it, so the correct code should be:

template<template<typename,typename> class x>
void test() {
    x<int, std::allocator<int>>();
}

template template arguments: no matching call found

2 Answers2