c++ - What is the reason for not being able to deduce array size from initializer-string in member variable? -

consider code:

struct foo {     const char str[] = "test"; };  int main() {     foo foo; } 

it fails compile both g++ , clang++, spitting out essentially

error: array bound cannot deduced in-class initializer

i understand standard says, there particular reason why? since have string literal seems compiler should able deduce size without problem, case when declare out-of-class const c-like null terminated string.

the reason have possibility override in class initializer list in constructor. guess in end confusing.

struct foo {    foo() {} // str = "test\0";     // implementing easier if can see how big `str` is,     foo() : str({'a','b', 'c', 'd'}) {} // str = "abcd0"    const char str[] = "test"; }; 

notice replacing const char static constexpr works perfectly, , want anyway