Why can char be initialized to nullptr in a std::array, but not by itself?

Question

This code doesn't compile (using gcc 9.3)...

int main() {
    char bar = nullptr; //error: cannot convert ‘std::nullptr_t’ to ‘char’ in initialization
}

But this code does compile...

#include <array>
int main() {
    std::array<char, 1> foo = {nullptr}; // foo[0] == char(0), why?
}

Why is there a distinction?

The 2nd one doesn't compile on my machine. JakeSchmidt.cpp:3:32: error: cannot initialize an array element of type 'char' with an rvalue of type 'nullptr_t. — Eljay
– Eljay, Commented May 26, 2020 at 1:35
Okay -- mystery solved. I have gcc 7 and 10 to test on. Seems there was some sort of regression in gcc 9. — David C. Rankin
– David C. Rankin, Commented May 26, 2020 at 1:59

Remy Lebeau · Accepted Answer · 2020-05-26 07:38:23Z

5

Why can char be initialized to nullptr in a std::array

It can't. The shown program is ill-formed in C++.

When an ill-formed program compiles, there are typically two possibilities:

In this case, I think it is the latter. The bug reproduces in GCC 9, but appears to have been fixed in GCC 10.

Remy Lebeau

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answered May 26, 2020 at 1:59

eerorika

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