I want to create objects of a class with user defined size at run time. Like when user enter 5 it creates 5 objects. I tried this
student s[num];
for(int i=0;i<num;i++)
{
student s[i]; // also used student s[i]=new student();
}
but it says student s[num] expression must have constant value.
So how to do this?
A std::vector was made for just this purpose. (basically a dynamically allocated array) It has a constructor taking the number of elements to pre-allocate and construct:
#include <vector>
std::vector<student> students(num);
You can loop over it as a normal array:
for(unsigned int i = 0; i < students.size(); ++i)
{
//students[i]..
}
Or, if you have access to C++11 use a range-based loop:
for(auto const& student : students)
{
//student..
}
Or, it's iterators:
for(std::vector<student>::iterator itr = students.begin(); itr != students.end(); ++itr)
{
//access with *itr..
}
students.at(i) instead of students[i], in order to get run-time bounds checking.
i < students.size() so [i] must be valid.[] on vectors. It's a bad habit to allow it. If I was teaching C++, I would never even mention it's existence! :-)[] and always using .at(), then former will definitely introduce undefined behaviour into every non-trivial codebase. I will never believe that anybody, even the smartest person, can avoid all incorrect usage of []. So the overhead is definitely not "pointless" in generalAlternatively, you can dynamically allocate and deallocate. If you new[] don't forget to delete[]!
#include <iostream>
struct MyObj {
float a;
float b;
};
int main() {
int num = 10;
MyObj* objs = new MyObj[num];
for(int i = 0; i < num; i++) {
objs[i].a = i;
objs[i].b = i*2;
}
std::cout << objs[3].a << " * 2 = " << objs[3].b << std::endl;
delete [] objs;
return 0;
}
std::vector<student> s(num);ArrayList.#include <vector>Tricky