I'm designing a class that has a std::vector<int> as an instance variable. I'm using a std::vector because I need to set its size at runtime. Here are the relevant portions of my code:
my_class.h:
#include <vector>
using std::vector;
class MyClass {
int size;
vector<int> vec;
}
my_class.cc:
#include "my_class.h"
using std::vector
MyClass::MyClass(int m_size) : size(m_size) {
vec = new vector<int>(size,0);
}
When I attempt to compile I get these error messages:
g++ -c -Wall my_class.cc -o my_class.o
my_class.cc: In constructor ‘MyClass::MyClass(int):
my_class.cc:4 error: no match for ‘operator=’ in ‘((MyClass*)this)->My_Class::vec = ((*(const allocator_type*)(& std::allocator<int>())), (operator new(24u), (<statement>, ((std::vector<int>*)<anonymous>))))’
make: *** [my_class.o] Error 1
However, when I change the offending line to:
vector<int> temp(size,0);
vec = temp;
It now compiles without a hitch and I get the desired behavior and can access my vector as
vec[i] // i having been defined as an int yada yada yada
This workaround is okay, but I would like to understand why it works and the first method fails.
Just do:
MyClass::MyClass(int m_size) : size(m_size), vec(m_size, 0)
You already seem to know about initializer lists, why not initialize vector there directly?
vec = new vector<int>(size,0);
is illegal because new returns a pointer and in your case vec is an object.
Your second option:
vector<int> temp(size,0);
vec = temp;
although it compiles, does extra work for no gain. By the time you reach the assignment, two vectors would already have been constructed and discarded afterwards.