if

Introduces an if statement. The keyword if must be followed by a parenthesized condition, which can be either an expression or a declaration. If the condition is truthy, the substatement after the condition will be executed.

int x;
std::cout << "Please enter a positive number." << std::endl;
std::cin >> x;
if (x <= 0) {
    std::cout << "You didn't enter a positive number!" << std::endl;
    abort();
}