Question

I have some code:

const string MY_STRINGS[3] = { "A", "B", "C" };

and when I try to do this:

int main() {
  MY_STRINGS[1] = MY_STRINGS[2];
}

I get the following error:

Passing ‘const std::string’ as ‘this’ argument of ‘std::basic_string… & std::basic_string::operator= …’ discards qualifiers.

Sure. Makes sense. I can’t assign to a const string.

But when I do the following:

int main() {
  const string SOME_STRINGS[3] = MY_STRINGS;
  MY_STRINGS[1] = MY_STRINGS[2];
}

this compiles and works. What has changed, and why does it now compile?

Answer 1

I believe GCC copies the array. Thus ensuring the const array is immutable and the variable array mutable.

#include <string>
#include <iostream>

using namespace std;

int main() {
    string varStrs[] = {"unchanged", "changed"};
    const string constStrs[2] = varStrs;

    varStrs[0] = varStrs[1];
    //constStrs[0] = constStrs[1]; // <-- will not compile

    cout << "const str: " << constStrs[0] << endl;
    cout << "var str: " << varStrs[0] << endl;

    return 0;
}

I tested this on GCC version 4.1.2