C++ Free Storage (Heap, Dynamic Allocation ...)


The term 'heap' is a general computing term meaning an area of memory from which portions can be allocated and deallocated independently of the memory provided by the stack.

In C++ the Standard refers to this area as the Free Store which is considered a more accurate term.

Areas of memory allocated from the Free Store may live longer than the original scope in which it was allocated. Data too large to be stored on the stack may also be allocated from the Free Store.

Raw memory can be allocated and deallocated by the new and delete keywords.

float *foo = nullptr;
    *foo = new float; // Allocates memory for a float
    float bar;              // Stack allocated 
} // End lifetime of bar, while foo still alive

delete foo;          // Deletes the memory for the float at pF, invalidating the pointer
foo = nullptr;       // Setting the pointer to nullptr after delete is often considered good practice

It's also possible to allocate fixed size arrays with new and delete, with a slightly different syntax. Array allocation is not compatible with non-array allocation, and mixing the two will lead to heap corruption. Allocating an array also allocates memory to track the size of the array for later deletion in an implementation-defined way.

// Allocates memory for an array of 256 ints
int *foo = new int[256];
// Deletes an array of 256 ints at foo
delete[] foo;

When using new and delete instead malloc and free, the constructor and destructor will get executed (Similar to stack based objects). This is why new and delete are prefered over malloc and free.

struct ComplexType {
    int a = 0;

    ComplexType() { std::cout << "Ctor" << std::endl; }
    ~ComplexType() { std::cout << "Dtor" << std::endl; }

// Allocates memory for a ComplexType, and calls its constructor
ComplexType *foo = new ComplexType();
//Calls the destructor for ComplexType() and deletes memory for a Complextype at pC
delete foo;

From C++11 on, the use of smart pointers is recommended for indicating ownership.


C++14 added std::make_unique to the STL, changing the recommendation to favor std::make_unique or std::make_shared instead of using naked new and delete.