Question

I have something like this:

#include <iostream>
#include <map>

int main() {

    std::map<int, int*> mapaString;
    int* teste = mapaString[0];
    std::cout << teste << std::endl;
    if(!teste)
        mapaString[0] = new int(0);

    std::cout << mapaString[0] << std::endl;
    std::cout << mapaString[1] << std::endl;

    return 0;
}

In documentation at gcc and cpluplus.com it’s just said that will be called the default constructor of the element, but when a pointer is declared without initializing it, its value will be undefined.

Is it guaranteed that the value returned will be a NULL pointer when calling subscript operator([]) when there is no mapped value assigned to the key and return type is a pointer?

Answer 1

The “default constructors” of primitive types (including pointers) produce 0-filled memory, much like global variables.

Here is the relevant standard language (from dcl.init):

To default-initialize an object of
type T means:

–if T is a non-POD class type
(class), the default constructor for
T is called (and the initialization is ill-formed if T has
no acces-
sible default constructor);

–if T is an array type, each
element is default-initialized;

–otherwise, the storage for the
object is zero-initialized.

…

7 An object whose initializer is an
empty set of parentheses, i.e., (),
shall be default-initialized.

Also, from lib.map.access:

23.3.1.2 map element access [lib.map.access]

reference operator[](const key_type&
x);

Returns:
(*((insert(make_pair(x, T()))).first)).second.