What are issues if we mix new and free in C++?

Answer: new and free in c++ should never be mixed or else we may get memory leak issues and resource leak issues and may take extra effort and time to maintain the C++ software project.Memory leak really a not a good elements for software and cause poor performance to a software.

Recommended interview question read: What are issues due to memory leaks in C++ software applications?

1 – Memory & resource leak


We should stick to use new and delete for dynamic memory allocation and de-allocation respectively in C++ program. Mixing new and free is not a good idea, as C++ new operator calls the constructor and delete operator calls destructor of a class.
If we use free, it will not call destructor of a class. You might have made some de-allocation and resource clean-up in class destructor. So, memory and resources will be leaked, that will drastically affect performance of the application.

Sample code:

#include 
using namespace std;

class A{
private:
	int *p;

public:
	A(){
		cout<<"Constructor!!!\n";
		p = new int;*p=5;
	}
	~A(){
		cout<<"Destructor!!!,Cleaning resources\n";
		//resource clean-up
		cleanup();
	}
	void cleanup(){
		cout<<"resource cleanup done!!!"<<endl;
	}
	void display(){
		cout<<"value is: "<<*p<<"\n"; 	} }; int main(){ 	A *ptr = new A(); 	ptr->display();
	delete ptr;// calls destructor, hence resource cleanup

	A *ptr1 = new A();
	ptr1->display();
	free(ptr); // no destructor will be called, so, no resource cleanup


	return 0;
}
Output:
       Constructor!!!
       value is: 5
       Destructor!!!,Cleaning resources
       resource cleanup done!!!
       Constructor!!!
       value is: 5

In the output, notice that when we call delete ptr, destructor gets called and resource clean up is happening while in case of free(ptr)there is no destructor call hence missing resource clean up.

2 – May require extra efforts and time during project development or maintenance.

If no memory de-allocation and resource clean-up is happening in destructor of a class in C+, then we can use free() function instead of delete operator in C++. But, as a best practice we should always use delete operator to deallocate memory, so it can call destructor of a class.

Assume, at some later point of time during project development and maintenance, if some other new programmer comes and put memory de-allocation and resource clean-up code in destructor of the class then application will be in trap as on using free() desturctor will not be called resulting no cleanup. A programmer has to find all related free () functions in the project and replace with delete operator in c++ and test.

So, avoid extra efforts and time by not mixing new and free in C++ program.

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