Local type as template arguments in C++

Question

This is my code

#include <vector>
template <typename T, template<typename> class C = std::vector >
struct FooBar
{
   /*codez*/
};
template<typename T>
struct Global{};

int main()
{
   struct Local{};  
   FooBar<Local,Global> k;
}

This is the error that I get

template argument for ‘template<class T, template<class> class C> struct FooBar’ uses local type ‘main()::Local’

Which part of the standard says that this is wrong? I am using gcc 4.5.1. How can make this code work?

Ah, that ability doesn't come standard until C++0x. See N2657 ( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm ). Just add that option and g++ will be happy with you. — Travis Gockel, Apr 22 '11 at 03:37
https://en.cppreference.com/w/cpp/language/class#Local_classes : "local classes could not be used as template arguments (until C++11) " — jw_, Dec 30 '19 at 09:47

score 33 · Accepted Answer · edited Jan 19 '15 at 16:45

33

Which part of the standard says that this is wrong?

That would be §14.3.1/2 from the 2003 C++ Standard:

A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template type-parameter.

How can make this code work?

Don't use a local type as a template argument.

Note that this restriction has been lifted in C++11, so using that language standard you are able to use a local type as a template argument.

edited Jan 19 '15 at 16:45

Columbo

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answered Apr 22 '11 at 03:26

James McNellis

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I used `-std=c++0x` option and the code works. However if I use `FooBar k;` instead of `FooBar k;` I get some errors. What's wrong with `FooBar k`? – Ron Kanga Apr 22 '11 at 03:38
I'm glad to hear that g++ supports that feature. As for the problem with `FooBar`, it is because `std::vector` is not a valid template template argument for `template class` because `std::vector` isn't declared as `std::vector`, it's declared as `std::vector`. – James McNellis Apr 22 '11 at 03:39
@Ron: `std::vector` has _two_ template arguments. So, `template class C = std::vector> struct FooBar { .. };` See it working [here](http://www.ideone.com/9FQZP). – Lightness Races in Orbit Apr 22 '11 at 03:41

score 8 · Answer 2 · answered Apr 22 '11 at 03:28

14.3.1/2

A local type, a type with no linkage, an unnamed type or a type compounded from any of these types shall not be used as a template-argument for a template type-parameter.

Try compiling your code with -std=c++0x option (gcc 4.5.1). C++0x lifts the restriction and so you can use a local type as template argument.

Local type as template arguments in C++

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