Can C \'s \'if\' Statement Handle Multiple Variable Declarations?

Variable Declarations in "if" Expressions: Standards and Compiler Quirks

The Problem

In C , variable declarations within "if" statement conditions have been a subject of confusion and compiler limitations. Code snippets like the following often raise questions:

if (int a = Func1()) { ... } // Works
if ((int a = Func1())) { ... } // Fails to compile
if ((int a = Func1()) && (int b = Func2())) { ... } // Works

The C 03 standard allows variable declarations in "if" conditions, with scope extending to the end of the substatements controlled by the condition. However, it doesn't specify restrictions on parentheses or multiple declarations.

The Limitation

Despite the standard's silent allowance, many compilers, including VS2008, enforce limitations:

• Parentheses cannot enclose the declaration.
• Only one declaration is permitted per condition.

A More Annoying Example

This limitation can be particularly annoying when declaring multiple variables within a condition and assigning specific values to them. Consider:

bool a = false, b = true;
if (bool x = a || b) { ... }

To enter the "if" body with x set to false, parentheses are required around the declaration. However, since parentheses are disallowed, x must be declared outside the body, leaking it to a wider scope.

In Conclusion

Before C 17, the desired syntax for multiple variable declarations in "if" conditions was not conformant to the standard. However, compilers often imposed further limitations.

The C 17 Solution

Thankfully, C 17 introduced a new syntax that resolves this issue:

if (int a = Func1(), b = Func2(); a && b) { ... }

In C 17, you can now use a semicolon to separate the declaration from the actual condition, allowing for multiple variable declarations and parentheses.

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