GCC shared_ptr Template Error

Question

The following function

#include <memory>

template<typename T>
std::shared_ptr<typename T> Tail(const std::shared_ptr<typename T>& cont, size_t n)
{
   const auto size(std::min<size_t>(n, cont->size()));
   return std::shared_ptr<typename T>(new T(cont->end() - size, cont->end()));
}

Produces the following error on gcc 4.7.2.

g++ test.cpp -std=c++0x
test.cpp:4:27: error: template argument 1 is invalid
test.cpp:4:66: error: template argument 1 is invalid
test.cpp: In function ‘int Tail(const int&, size_t)’:
test.cpp:6:42: error: base operand of ‘->’ is not a pointer
test.cpp:7:35: error: template argument 1 is invalid
test.cpp:7:47: error: base operand of ‘->’ is not a pointer
test.cpp:7:67: error: base operand of ‘->’ is not a pointer

I understand that cont does not 'look' like a pointer, but this compiles fine on VS2012. How can I write the function for gcc?

score 3 · Answer 1 · answered Mar 26 '13 at 07:30

3

Just remove those extra typenames

template<typename T>
std::shared_ptr<T> Tail(const std::shared_ptr< T>& cont, size_t n)
{
   const auto size(std::min<size_t>(n, cont->size()));
   return std::shared_ptr< T>(new T(cont->end() - size, cont->end()));
}

answered Mar 26 '13 at 07:30

masoud

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Thanks for the answer, why is that? – Craig Mar 26 '13 at 07:32
Simply you don't need them – masoud Mar 26 '13 at 07:33

score 2 · Answer 2 · edited May 23 '17 at 12:21

2

You are overusing the typename keyword. The code should look as follows:

template<typename T>
std::shared_ptr<T> Tail(const std::shared_ptr<T>& cont, size_t n)
{
   const auto size(std::min<size_t>(n, cont->size()));
   return std::shared_ptr<T>(new T(cont->end() - size, cont->end()));
}

For further discussion, see Where and why do I have to put the "template" and "typename" keywords?

edited May 23 '17 at 12:21

Community

1
1

answered Mar 26 '13 at 07:31

NPE

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score 0 · Answer 3 · answered Mar 26 '13 at 07:32

you don't have to rewrite typename before every use of arguments, so please change it to:

template<typename T>
std::shared_ptr<typename T> Tail(const std::shared_ptr<T>& cont, size_t n)
{
   const auto size(std::min<size_t>(n, cont->size()));
   return std::shared_ptr<typename T>(new T(cont->end() - size, cont->end()));
}

this is the same issue as with i.e. classes, you don't write void myfunc(class MyClass &m){} but simply void myfunc(MyClass &m){}

GCC shared_ptr Template Error

3 Answers3