Local type as template arguments in C++ [duplicate]

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 ( open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm ). Just add that option and g++ will be happy with you. — Travis Gockel
– Travis Gockel, Commented Apr 22, 2011 at 3:37
en.cppreference.com/w/cpp/language/class#Local_classes : "local classes could not be used as template arguments (until C++11) " — jw_
– jw_, Commented Dec 30, 2019 at 9:47

Columbo · Accepted Answer · 2015-01-19 16:45:47Z

34

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, 2015 at 16:45

Columbo

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answered Apr 22, 2011 at 3:26

James McNellis

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3 Comments

Ron Kanga Over a year ago

I used -std=c++0x option and the code works. However if I use FooBar<Local> k; instead of FooBar<Local,Global> k; I get some errors. What's wrong with FooBar<Local> k?

James McNellis Over a year ago

I'm glad to hear that g++ supports that feature. As for the problem with FooBar<Local>, it is because std::vector is not a valid template template argument for template <typename> class because std::vector isn't declared as std::vector<T>, it's declared as std::vector<T, Alloc>.

Lightness Races in Orbit Over a year ago

@Ron: std::vector has two template arguments. So, template <typename T, template<typename, typename> class C = std::vector> struct FooBar { .. }; See it working here.

Prasoon Saurav · Accepted Answer · 2011-04-22 03:28:00Z

8

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.

answered Apr 22, 2011 at 3:28

Prasoon Saurav

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