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The stack is a small region of memory into which temporary values are placed during execution. Allocating data into the stack is very fast compared to heap allocation, as all the memory has already been assigned for this purpose.

int main() {
    int a = 0; //Stored on the stack
    return a;

The stack is named because chains of function calls will have their temporary memory ‘stacked’ on top of each other, each one using a separate small section of memory.

float bar() {
    //f will be placed on the stack after anything else
    float f = 2;
    return f;

double foo() {
    //d will be placed just after anything within main()
    double d = bar();
    return d;

int main() {
    //The stack has no user variables stored in it until foo() is called
    return (int)foo();

Data stored on the stack is only valid so long as the scope that allocated the variable is still active.

int* pA = nullptr;

void foo() {
    int b = *pA;
    pA = &b;

int main() {
    int a = 5;
    pA = &a;
    //Undefined behavior, the value pointed to by pA is no longer in scope
    a = *pA;

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