STL vector initial value

Which datatypes are you referring to? Built-in ones? In that case, the vector will use T() as the initializer, e.g., int(), double() etc. The effect of such an expression:

int n= int();

is default initializing n, which in the case of fundamental types means zero initialization, or initialization with zero converted to the target type, e.g., false, null pointer value etc.

If you want to be 100% sure about the default values, you can use a more explicit form:

vector<double> val(12, 0.0); //allocate 12 doubles initialized to 0.0