c++11 constexpr function cannot receive "const" return value?

Question

New to c++11 constexpr, I'm using latest clang++-14 version on windows and try to see how const and constexpr could work together.

I've got a global variable of char* and wish to return it, in a constexpr function, as below:

char* pHello = "hello";
constexpr char* hello() {
    return pHello; // compilation error
}

It doesn't compile, clang says:

Constexpr function never produces a constant expressionclang(-Winvalid-constexpr)
xxxcpp(20, 12): Read of non-constexpr variable 'pHello' is not allowed in a constant expression
xxx.cpp(18, 7): Declared here

Then in the first line, I added a const:

const char* pHello = "hello"; // add const here
constexpr char* hello() {
    return pHello; // still compilation error
}

This time, the clang error became:

Cannot initialize return object of type 'char *' with an lvalue of type 'const char *'clang(init_conversion_failed)

Reading this error message, I tried to modify the hello() signature as:

constexpr const char* hello() {

Now the error became same as original:

Constexpr function never produces a constant expressionclang(-Winvalid-constexpr)
xxx.cpp(20, 12): Read of non-constexpr variable 'pHello' is not allowed in a constant expression
xxx.cpp(18, 13): Declared here

I make this test just to see how constexpr and const would work together, but even when I change the global variable as also constexpr like this:

constexpr char* pHello = "hello";
constexpr char* hello() {
    return pHello;
}

Still doesn't compile:

error: cannot initialize return object of type 'char *' with an lvalue of type 'const char *'
    return pHello;

How to make it work?

Nimrod · Accepted Answer · 2022-08-17 09:51:02Z

1

The type of pHello is const char* (a pointer to const char), and add constexpr to make it a constant expression.

The return type of function hello should be consistent with pHello, you can't lose constness here. constexpr here is only used to mark this function a constexpr function.

constexpr const char* pHello = "hello";
constexpr const char* hello() {
    return pHello;
}

Demo

edited Aug 17, 2022 at 9:51

answered Aug 17, 2022 at 9:45

Nimrod

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