Why does the `equal` works for const char* in C++?

问题

The codes are like this, it outputs 1:

int main(int argc, char *argv[])
{
    vector <const char*> vec = {"nima","123"};
    vector <const char*> vec2 = {"nima","123"};
    auto result = equal(vec.cbegin(), vec.cend(), vec2.cbegin());
    cout << result << endl;
    return 0;
}

I knew that I can test whether two c-style string is equal only by using strcmp (because char* is not an object as I understood). But here equal is a function from <algorithm>. Does it overload the == operator so that it can test the equality of two char*?

@Damon says that they're equal as the merges the same string literal into same address, as I understood. However, when I tried char* with different addresses, it still gives me the same result:

int main(int argc, char *argv[])
{
char* k = "123";
char* b = "123";
vector <const char*> vec = {"nima"};
vector <const char*> vec2 = {"nima"};
cout << &k << endl;
cout << &b << endl;
vec.push_back(k);
vec2.push_back(b);

auto result = equal(vec.cbegin(), vec.cend(), vec2.cbegin());
cout << result << endl;
return 0;
}

The result is:

 0x7fff5f73b370
 0x7fff5f73b368
 1

回答1:

What probably happens here is that the compiler/linker merges the four string literals of which two are identical into two literals. Therefore, both "nima" and both "123" have the same address.

You store the addresses in a vector and compare them (operator== on an address). Since the addresses are the same, the comparison is equal.

Note that this is accidential. It only "works" because of two reasons:

1. The strings are literals (that is, not some strings read from e.g. stdin).
2. A compiler is allowed, but not required to merge identical literals (whether or not this happens is implementation-defined).

This can lead to two situations in which you get a funny surprise (not so funny if you must find out why it suddenly doesn't work when it "worked" all the time), either when you use a different compiler or even the same compiler with different optimization settings, or when you assign non-literal strings.

来源：https://stackoverflow.com/questions/14911085/why-does-the-equal-works-for-const-char-in-c