In the example code, compiled in C 17:

template <typename T = int>
struct A
{
    static constexpr double b = 0.5;
};

int main()
{
    A a;  // compiles
    double c = A<>::b; // compiles
    double d = A::b; // fails to compile
    // ...
    return 0;
}

Live example

A::b fails to compile because:

main.cpp:13:16: error: 'template<class T> struct A' used without template arguments
   13 |     double d = A::b; // fails to compile

I thought in C 17 automatic template type deduction would take care of this since I have the default template argument. What am I missing?

CodePudding user response：

Each instantiated class instance has own A<T>::b member.

#include <iostream>

template <typename T = int>
struct A
{
    static constexpr double b = 0.5;
};

int main()
{
    std::cout << &A<int>::b << "\n"; 
    std::cout << &A<char>::b << "\n"; 
    return 0;
}

Compiled with clang 13 with the option -std=c 17. Output:

0x402008
0x402018

Think about if A::b could be compiled, which address would be selected by the compiler.

The live example https://godbolt.org/z/GqhjMvoz6

As for failed to compile double d = A::b; with the default template parameter, this is the rule, templates must be instantiated used with using angle brackets <, >.