What's the difference between an empty string and a '\0' char? (from a pointer and array point of view)

Question

As the title of this question? What's the difference?

If I write:

char *cp = "a";
cout << (cp == "a") << endl;

or:

string str = "a";
cout << (str == "a") << endl;

They are the same and all return true. But if I write:

char *cp = "";
cout << (cp == "\0") << endl;

It returns false. But:

string str = "";
cout << (str == "\0") << endl;

It returns true.

I thought they should be the same from the pointer and array perspective. But they turn out to be different. What's the subtle difference between them? And how should I write a char array to represent an empty string?

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OK, what's above the line might be unclear like somebody said it might be "a compiler optimization".

What I really want is this:

char *cp = "abcd";

can give me an array like this: ['a', 'b', 'c', 'd', '\\0']. I was wondering how can I use the similar syntax to get an array like this: ['\\0']. Because I tried:

char *cp = "";

And it seems not the right code. I thought it could give me what I want. But it doesn't.

Sorry for the ambiguousness above the line. I'm a newbie and I don't the that might be a compiler optimization.

Answer 1

String literals always have an implicit \\0 at the end. So "" is of type const char[1] and consists of one \\0 , whereas "\\0" is of type const char[2] and consists of two \\0 s. If you want the empty string literal, just write "" . There is no need to insert another \\0 manually.

operator==(const char*, const char*) compares pointers, not characters. If you compare two different string literals with == , the result is guaranteed to be false. (If you compare two string literals consisting of the same characters with == , the result is not well-defined.)

std::string::operator==(const char*) treats the argument as a C string. That is, it will only read until it encounters the first \\0 . Hence, it cannot distinguish between "" and "\\0" .

Answer 2

The "\\0" actually contains two '\\0' characters, one of them created implicitly.

If you write something like

char *cp = "a";
cout << (cp == "a") << endl;

you are comparing two pointers, not the string contents (use strcmp() to do so), while

std::string cp = "a";
cout << (cp == "a") << endl;

compares the contents up to the first found '\\0' , because the bool std::string::operator==(const char*) is overloaded to do so.

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As for your edits:

Because I tried:

char *cp = ""; And it seems not the right code. I thought it could give me what I want. But it doesn't.

char* cp = "";

isn't exactly the same as

char cp[1] = { '\0' };

since the string literal is created with static storage location, while the array version allocates memory on the stack.