How does std::vector deal with variable length std::string?

Question

We can construct a vector to store a bunch of strings by writing vector, but a string is can be variable length, how can vector deal with that?
I also test a demo, test[0] begin with 0x2508cb0, test[1] begin with 0x2508cb8, but the diff of two addresses and the capacity of test[0] seems to be not same.

int main() {

vector<string> test;
test.push_back("tes3235235et");
test.push_back("135125151241241241");
cout << test[0].capacity() << endl;
cout << test[1].capacity() << endl;
cout << &(test[0]) << endl;
cout << &(test[1]) << endl;
return 0;

}

Output:

12
18
0x2508cb0
0x2508cb8

String objects all have the same size, it is the size of the data that is stored by string objects (and probably stored on the heap) may vary. — user7860670
– user7860670, Commented Nov 11, 2017 at 19:14

Benjamin Lindley · Accepted Answer · 2017-11-11 19:14:12Z

5

The vector doesn't need to deal with that, because the string deals with that. Just like std::vector, std::string stores its elements in dynamically allocated memory. The characters are not part of the string object itself (except in the case of small string optimization), but are instead just referred to via a pointer. The actual size of the string object is set at compile time and is the same for all strings (and can be obtained by sizeof(std::string)), regardless of the number of characters.

answered Nov 11, 2017 at 19:14

Benjamin Lindley

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1 Comment

underscore_d Over a year ago

"The vector doesn't need to deal with that, because the string deals with that." Right, and that's kinda the whole point of classes and objects. :)

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