Can a variable be defined only in the scope of an if-statement, similar to how it's often done for for-loops?

Question

Is there a way to declare, assign and compare a variable to an expression in an if construction in such a way that it is only defined in the scope of the if construction?

Answer 1

As far as I can tell there is no way to have both a declaration and an expression within the condition of an if statement. If we look at the draft C++ standard section 6.4 Selection statements the grammar for if is as follows:

selection-statement:
    if ( condition ) statement
    if ( condition ) statement else statement
    switch ( condition ) statement
condition:
    expression
    attribute-specifier-seqopt decl-specifier-seq declarator = initializer-clause
    attribute-specifier-seqopt decl-specifier-seq declarator braced-init-list

So you either can use an expression or a declaration and I don't see any obvious ways around that.

What you proposed in the alternative, declaring i before the if statement seems like the best option. Although using an enclosing block does not seem necessary:

int i = f();
if(i == 3)