CODEWITHSUNDEEP
c++memory-managementheap-memorystack-memory
nebulabrot
nebulabrot

Reputation: 475

C++ Allocating memory on the heap and stack?

Coming from a Java background, I'm still a little confused about allocating memory in C++. I'm pretty sure the first two statements are correct:

void method() {
    Foo foo;    // allocates foo on the stack, and the memory is freed
                // when the method exits
}

void method2() {
    Foo *foo = new Foo();   // allocates foo on the heap
    delete foo;             // frees the memory used by foo
}

But what about something like this?

void method3() {
    Foo foo = *new Foo();   // allocates foo on the heap, and then copies it to the stack?
                            // when the method exits, the stack memory is freed, but the heap memory isn't?
}

Say I added foo to a global array inside method3(). If I tried to access one of foo's data members after the method exits, would that work? And is method3() prone to memory leaks?

Thanks in advance.

Upvotes: 8

Views: 9926

Answers (1)

Alok Save
Alok Save

Reputation: 206498

Foo foo();

Declares a function by the name foo which returns a Foo object and does not take any arguments. It is known as the most vexing parse in C++. You probably meant:

Foo foo;

It creates a foo object locally/automatic storage. The object is automatically deallocated once the scope { } in which it is declared ends.

Foo *foo = new Foo();   // allocates foo on the heap
delete foo;

This is true, the object on freestore pointed by foo is deallocated once you call delete. There is no memory leak.

 Foo foo = *new Foo();

Allocates a Foo object on freestore and then a copy of that object is used to initialize foo. Since you do not have a pointer to the freestore allocated object, it causes a memory leak. Note that if the destructor of Foo has some code which causes side effects then it is not merely memory leak but undefined behavior.

Upvotes: 10

Related Questions