Local functions (a.k.a., nested 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. Therefore, local functions 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 lambda functions assigning them to local variables. For example (see also add_cxx11_lambda.cpp):

int main(void) {                            // 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_TEST(sum == 60);                  // Assert final summation value.
    return boost::report_errors();
}

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

int main(void) {                            // 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_TEST(sum == 60);                  // Assert final summation value.
    return boost::report_errors();
}

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.