There are two kinds of if statements in modern C++: runtime if and compile-time if constexpr.
Runtime if looks like:
In both forms of above if the result of condition yields to true, then statement-true is executed. Otherwise (in the second form of above) the statement-false is executed. Any expression that yields something convertible to bool in a context of if can be used as a condition, for example:
In the example above every couple of if statements performs the similar checks. In case 1 both if statements checks the number is not zero, while in case 2 both if statements checks the pointer contains a not null address that points to some memory area.
Logical operators && and || evaluates it’s second operand only if it is necessary, for example:
The example above demonstrates two cases: in case 3 *value is evaluated only if value yields to true, while in case 4 value.is_null() is evaluated only if !value yields to false. In other words operands of && only evaluates until the first false while operands of || only evaluates until the first true.
Please note, that there is no “else-if” statement in C++. Therefore, when it is necessary to specify multiple if branches (alternatives), then two if statements must be combined as shown below:
To make the code looks more compact and to avoid the usage of variables before assigning initial values to them it is recommended to introduce variables into the smallest scope possible, in particular, it is possible to declare variable in condition of if statement, for example:
In the example above the variable “err” is declared and the value assigned to it at runtime is evaluated and yielded to bool to check the if condition. The scope of variable (“err” in the example) that is initialized inside an if statement extends to the end of statements which are follows the conditions, i.e. to the end of statement-true and statement-false (if else part is specified).
Since C++17 it is possible to specify the expression for condition checking just after the variable declaration, for example:
Since C++17 it is possible to perform compile-time checks by using if constexpr statement which looks similar to runtime if except the presence of the keyword constexpr immediately after the if. The condition of if constexpr statement must be a compile-time expression, for example:
In the example above if the function to_string() is called with an integral argument, the only branch 2 will be included to the result of compilation which reduces the size of the resulting binary. By replacing if constexpr with if in this example this effect is also possible without constexpr because compilers like the latest generation of Embarcadero RAD Studio C++ compilers can optimize code that is not used (so called “unreachable code”). But with constexpr this behavior is guaranteed by the standard.
When using logical operators && and || in if constexpr statements please keep in mind that compile-time condition is always instantiated and must be valid as a whole. Therefore all the functions and operators specified in the condition of if constexpr must be available at compile-time. Consider:
The example above won’t compile in case when T represents std::string for instance, because the comparison operation (value == 3.1415) cannot be performed against the value of type std::string (neither runtime nor compile-time). To make this example to compile it should be refactored as follows:
As in case of runtime if it is possible to declare variables in a condition of if constexpr as well as specify an expression to be evaluated to perform the condition checking just after such a declaration. Both the declaration and the condition checking expression must be compile-time expressions, for example:
Please note that variable a is declared with using of constexpr keyword.