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18 Dynamic Memory

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Readme.md

18. Dynamic Memory Allocation

We've seen previously that variables and variable sizes need to be defined in advance. There are scenarios where it is hard to determine the variable size in advance, and would only know the size at runtime. Fortunately there are two keywords new and delete that allow us to allocate memory dynamically.

Allocating Memory

int* foo;
foo = new int[5];

In the example above, a pointer variable foo is defined wtih no value assigned. In the second line, the pointer variable foo is then allocated an array to be of size 5.

To access a value of the elements of the pointer variable, the index notation can be used.

cout << foo[0] ; // 0 (no value has been assigned yet)

foo is a pointer to the memory address of foo[0].

cout << foo     << "\n"; // 0x210fe70
cout << &foo[0] << "\n"; // 0x210fe70
cout << foo[0]  << "\n"; // 0

When the variable foo was allocated those 5 memory slots, the machine handles allocating them in neighboring locations.

cout << &foo[0] << "\n"; // 0x210fe70
cout << &foo[1] << "\n"; // 0x210fe74
cout << &foo[2] << "\n"; // 0x210fe78
cout << &foo[3] << "\n"; // 0x210fe7c
cout << &foo[4] << "\n"; // 0x210fe80

If the size of the variable needs to be changed, then doing so allocates new memory for that variable.

foo = new int[7];
cout << &foo[0] << "\n"; // 0x231d2a0

Catching Memory Errors

If you do not have enough space in memory to allocate, an error will be thrown. You may not want your program to completely stop in such a scenario, but catch the error and continue on to perform another function.

foo = new (nothrow) int [5];

if (foo == nullptr) {
  // error assigning memory. Take measures.
}
  • The nothrow expression can be used to mute errors during an attempt at memory allocation.
  • Your code can then proceed to check if the foo variable has been allocated memory by checking if it has a memory address with foo == nullptr.

De-allocating Memory

Dynamic array allocations and de-allocations have to be managed formally in the code. So you have to be mindful about de-allocating especially when memory usage can be high because memory won't be released until the program terminates. To de-allocate memory you can use the delete expression as follows:

// array allocation
int* foo;
foo = new int[5];

delete[] foo;

Reference