emplace_back causes link error on static constexpr member

问题

Why does emplace_back take a reference of the member that requires a definition? What is the difference between emplace_back(integer literal) and emplace_back(static constexpr integer member)?

If I switch to C++17, it compiles fine. I found that in C++17 static constexpr data members are implicitly inlined. Does it mean the compiler implicitly creates a definition for them?

Example code:

class base {
    int n;
public:
    base(int n):n(n) {}
};

struct base_trait {
    static constexpr int n = 1;
};

int main(void) {
    vector<base> v;
    v.emplace_back(1);  // ok
    v.emplace_back(base_trait::n);  // link error with -std=c++14, ok with -std=c++17
    return 0;
}

回答1:

As you said, emplace_back takes arguments by reference, so passing base_trait::n causes it to be odr-used.

an object is odr-used if its value is read (unless it is a compile time constant) or written, its address is taken, or a reference is bound to it;

Before C++17, that means the definiton of base_trait::n is required here. But since C++17 the behavior changed, for constexpr static data member the out-of-class definition is not needed again.

If a const non-inline (since C++17) static data member or a constexpr static data member (since C++11) is odr-used, a definition at namespace scope is still required, but it cannot have an initializer. This definition is deprecated for constexpr data members (since C++17).

A static data member may be declared inline. An inline static data member can be defined in the class definition and may specify an initializer. It does not need an out-of-class definition. (since C++17)

来源：https://stackoverflow.com/questions/50549359/emplace-back-causes-link-error-on-static-constexpr-member