Init-Statment 条件表达式 C++

Question

Can someone please explain to me initializations in if-statements?

This question here is almost the same as mine C++17 if statement with initializer but no condition but I don't feel the responses there were adequate.

Why does this work?

if( int x = 4 ){ 
  std::cout << x;
}

I was under the impression that initialization does not return a value. Indeed the code below is an error:

std::cout << ( int y = 7 );

Quoting from the Standard N4380, the syntax should be:

"if constexpr_opt ( init-statement_opt condition ) statement"

And quoting further along:

"An if statement of the form if constexpr_opt ( init-statement condition ) statement is equivalent to

{ init-statement if constexpr_opt ( condition ) statement }"

And also:

"[Note: An init-statement ends with a semicolon. —end note]"

I can imagine the "int x=4" is the init-statement. But

• then the condition statement, in the example I gave, is missing and the condition statement is not marked as optional
• and the init-statement does not end in a semicolon .

EDITED : removed an incorrect statement.

Answer 1

This is not new C++17 syntax. It has always been there.

int x = 4 in your first example is not the init-statement , but the condition in the grammar rule you quoted.

condition can be either an expression or a declaration with initializer (and a few other restrictions). If it has the latter form, then the declaration introduces the declared variable in the branches of the if and the first branch is chosen if the declared variable, contextually converted to bool , yields true after its initialization.