Is this valid C++ code? This doesn't work how it supposed to be

Question

int main() {
    
    string str[5] = "ABCD";
    std::cout << str[3] << std::endl;
    std::cout << str[0] << std::endl;
    return 0;
}

This code prints:

ABCD

ABCD

I didn't get it, how str[3] prints ABCD?

Compiler: GCC 6.3

Answer 1

The code is not valid C++ code and it shouldn't compile. And it doesn't with clang and gcc version above 7. It is most likely a bug in older version of gcc that got fixed in version 7.

std::string str[5]

What you have here is a C array of 5 elements of std::string. This is how you would initialize it:

std::string strings[5] = {"1st string", "2nd string", "3rd string", "4th string", "5th string"};

In this case strings[0] would be "1st string" and strings[3] would be "4th string".

However don't do this. Don't use C arrays in C++. Use std::vector or std::array if you need an array of strings: std::array<std::string> strings or std::vector<std::string> strings.

That being said, I suspect that you want just one string, aka:

std::string str = "ABCD";

In this case str[0] is 'A' and str[3] is 'D'

Answer 2

You can try this to get your code compile:

string str[5] = {"ABCD", "", "", "", ""};

str[3] in your code means 4the string of the array if you get it compiled.

But you most likely meant to have:

char str[5] = "ABCD";

string and char are not the same thing. If you want to access single char in a string you don’t need an array of strings. A single string is all you need:

string str = "ABCD";

std::cout << str[3] << std::endl;

Answer 3

What's happening is:

the line string str[5] = "ABCD"; is simply copying/initializing all the indexes of variable str with the value ABCD like str[0] = "ABCD", str[1] = "ABCD", and so on.

So, when you run std::cout << str[3] << std::endl; and std::cout << str[0] << std::endl;, you're getting the value from the respective index of the string array str.