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c++c++20gcc-warninglikely-unlikely

How to use C++20's likely/unlikely attribute in if-else statement

This question is about C++20's [[likely]]/[[unlikely]] feature, not compiler-defined macros.

This documents (cppreference) only gave an example on applying them to a switch-case statement. This switch-case example compiles perfectly with my compiler (g++-7.2) so I assume the compiler has implemented this feature, though it's not yet officially introduced in current C++ standards.

But when I use them like this: if (condition) [[likely]] { ... } else { ... }, I got a warning:

"warning: attributes at the beginning of statement are ignored [-Wattributes]".

So how should I use these attributes in an if-else statement?

Solution

  • Based on example from Jacksonville’18 ISO C++ Report the syntax is correct, but it seems that it is not implemented yet:

    if (a>b) [[likely]] {

    10.6.6 Likelihood attributes [dcl.attr.likelihood] draft