In which cases is the copy constructor called, if a container is copied
c++, constructor, stl
Solution
In the first case, `a` needs to grow when you assign to it, which means that all its elements must be reallocated (and therefore destructed and constructed).
In the second case, `a` does not need to grow, hence the assignment operator is used.
See http://ideone.com/atPt9; adding an overloaded copy assignment operator that prints a message, we get the following for the second example:
parameterised constructor
copy constructor
copy constructor
copy constructor
parameterised constructor
copy constructor
copy constructor
copy constructor
a = b
copy assignment
copy assignment
copy assignment
Problem
I tested the following code: ``` #include <iostream> #include <vector> class foo { public: int m_data; foo(int data) : m_data(data) { std::cout << "parameterised constructor" << std::endl; } foo(const foo &other) : m_data(other.m_data) { std::cout << "copy constructor" << std::endl; } }; main (int argc, char *argv[]) { std::vector<foo> a(3, foo(3)); std::vector<foo> b(4, foo(4)); //std::vector<foo> b(3, foo(4)); std::cout << "a = b" << std::endl; a = b; return 0; } ``` I get ``` parameterised constructor copy constructor copy constructor copy constructor parameterised constructor copy constructor copy constructor copy constructor copy constructor a = b copy constructor copy constructor copy constructor copy constructor ``` However, if I replace `std::vector<foo> b(4, foo(4));` by `std::vector<foo> b(3, foo(4));` the copy constructor is not called by `a = b` and the output is ``` parameterised constructor copy constructor copy constructor copy constructor parameterised constructor copy constructor copy constructor copy constructor a = b ``` Why is in this case the copy constructor not called? I'm using g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1