c++11: Why a in-class initialization of a static constexpr not a definition?

Question

Consider the simple following:

class Test {
public:
    static constexpr int a = 1;
}

This is not considered a definition and hence needs the below definition outside of the class.

constexpr int Test::a;

Why?

what was the reason for C++11 behaves like that? I am learning c++ but finding that c++ is pretty messy!! — yapkm01
Before inline variables, all static member variables had to be defined in a single TU, to avoid breaking ODR. C++17 introduced inline variables, which makes this problem go away. — NathanOliver
See softwareengineering.stackexchange.com/questions/145299 for a detailed explanation. — cigien

eerorika eerorika · Accepted Answer · 2021-09-14T02:25:10

Why a in-class initialization of a static constexpr not a definition?

Because of One Definition Rule (ODR). The rule says that there must be exactly one definition of each non-inline non-member and static member variable. Class definitions, due to their nature, are typically included into multiple translation units. If class definition contained a variable definition, then inclusion into multiple translation units would violate the ODR.

Since C++17, the language has inline variables, so you can define such inline variables within class definitions.

c++11: Why a in-class initialization of a static constexpr not a definition?

1 Answers