Local functions are a form of information hiding and they are useful for dividing procedural tasks into subtasks which are only meaningful locally, avoiding cluttering other parts of the program with functions, variables, etc unrelated to those parts. Local functions therefore complement other structuring possibilities such as namespaces and classes. Local functions are a feature of many programming languages, notably Pascal and Ada, yet lacking from C++03 (see also [N2511]).

Using C++11 lambda functions, it is possible to implement local functions by naming the lambdas assigning them to local variables. For example (see also add_lambda.cpp):

{                                           // Some local scope.
    int sum = 0, factor = 10;               // Variables in scope to bind.

    auto add = [factor, &sum](int num) {    // C++11 only.
        sum += factor * num;
    };

    add(1);                                 // Call the lambda.
    int nums[] = {2, 3};
    std::for_each(nums, nums + 2, add);     // Pass it to an algorithm.

    BOOST_CHECK( sum == 60 );               // Assert final summation value.
}

This library allows to program local functions portably between C++03 and C++11 (and with performances comparable to lambdas on C++11 compilers, see the Alternatives section). For example (see also add.cpp):

{                                           // Some local scope.
    int sum = 0, factor = 10;               // Variables in scope to bind.

    void BOOST_LOCAL_FUNCTION(const bind factor, bind& sum, int num) {
        sum += factor * num;
    } BOOST_LOCAL_FUNCTION_NAME(add)

    add(1);                                 // Call the local function.
    int nums[] = {2, 3};
    std::for_each(nums, nums + 2, add);     // Pass it to an algorithm.

    BOOST_CHECK( sum == 60 );               // Assert final summation value.
}

This library supports the following features for local functions:

See the Alternatives section for a comparison between this library, C++11 lambda functions, Boost.Phoenix, and other C++ techniques that implement features related to local functions.