never seen it not being used (or its ancestor boost::function) when doing consulting, a large amount of large projects used by other projects use it (boost, qt,

never seen it not being used (or its ancestor boost::function) when doing consulting, a large amount of large projects used by other projects use it (boost, qt, llvm, eigen, opencv, the VST API, KDE ...). In particular most GUI toolkits have at least an equivalent as it's the only way to store callbacks of different types with an associated arbitrary state (e.g. lambdas)