Boost C++ Libraries

This section illustrates an alternative syntax for compilers without variadic macro support.

Sequence Syntax

Most modern compilers support variaid macros (notably, these include GCC, MSVC, and all C++11 compilers). However, in the rare case that programmers need to use this library on a compiler without variadic macros, this library also allows to specify its macro parameters using a Boost.Preprocessor sequence where tokens are separated by round parenthesis ():

(token1) (token2) ... // All compilers.

Instead of the comma-separated list that we have seen so far which requires variadic macros:

token1, token2, ... // Only compilers with varidic macros.

For example, the following syntax is accepted on all compilers with and without variadic macros (see also add_seq.cpp):

int main(void) {
    int sum = 0, factor = 10;

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

    add(1);
    int nums[] = {2, 3};
    std::for_each(nums, nums + 2, add);

    BOOST_TEST(sum == 60);
    return boost::report_errors();
}

However, on compilers with variadic macros the comma-separated syntax we have seen so far is preferred because more readable (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();
}

Note that the same macros accept both syntaxes on compilers with variadic macros and only the sequence syntax on compilers without variadic macros. Finally, an empty local function parameter list is always specified using void on compilers with and without variadic macros:

int BOOST_LOCAL_FUNCTION(void) { // No parameter.
    return 10;
} BOOST_LOCAL_FUNCTION_NAME(ten)

BOOST_TEST(ten() == 10);

Examples

For reference, the following is a list of most of the examples presented in this documentation reprogrammed using the sequence syntax instead of the comma-separated syntax (in alphabetic order):