CODEWITHSUNDEEP
c++gccclanglanguage-lawyerc++20
Karen Baghdasaryan
Karen Baghdasaryan

Reputation: 2271

Disagreement between GCC and clang about "typename" keyword

I was compiling the following code and GCC seems to accept the following code.

#include <map>

template<typename K, typename V>
class my_map {
private:
    std::map<K, V> mmap;
public:
    typedef std::map<K, V>::iterator iterator;
    typedef std::map<K, V>::const_iterator const_iterator;

    iterator begin() {return mmap.begin();}
    const_iterator begin() const {return mmap.begin();}

};

int main()
{
    my_map<int, int>::iterator whatever;
    return 0;
}

But clang complains about the missing typename keyword. The complaint of clang makes more sense to me. Is this a GCC bug?

Edit: Obviously, Where and why do I have to put the "template" and "typename" keywords? does not answer my question, as I am not asking what typename keyword is, but asking about different behaviors of compilers.

Edit: Now the program is accepted by both clang(from version 16) and gcc.

Upvotes: 2

Views: 351

Answers (1)

user12002570
user12002570

Reputation: 1

asking about different behaviors of compilers.

It seems that Clang has not implemented this C++20 feature yet. This can be seen from compiler support documentation. This means clang is not standard compliant.

C++20 feature Paper(s) GCC Clang MSVC Apple Clang
Allow lambda-capture [=, this] P0409R2 8 6 19.22* 10.0.0*
Make typename more optional P0634R3 9 19.29 (16.10)*
Pack expansion in lambda init-capture P0780R2 9 9 19.22* 11.0.3*

As we can see in the above above table, the entry for clang corresponding to "Make typename more optional" is blank.

On the other hand GCC and MSVC have implemented this C++20 feature and so are standard compliant. GCC and MSVC Demo

Update

As of 03/11/2022, clang supports this with version 16 as shown in the updated table below:

C++20 feature Paper(s) GCC Clang MSVC Apple Clang
Allow lambda-capture [=, this] P0409R2 8 6 19.22* 10.0.0*
Make typename more optional P0634R3 9 16 19.29 (16.10)*
Pack expansion in lambda init-capture P0780R2 9 9 19.22* 11.0.3*

The program compiles with clang trunk.

Upvotes: 2

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