Example C++ function
Let us start with a simple C++ function which we wish to make available to C code:
namespace outcome_e = BOOST_OUTCOME_V2_NAMESPACE::experimental;
// Fill the supplied buffer with the integer v converted to a string,
// returning length of string minus null terminator
outcome_e::status_result<size_t> to_string(char *buffer, size_t bufferlen, int v) noexcept
{
try
{
// Could throw an exception!
std::string temp(std::to_string(v));
// Will this string exceed the supplied buffer?
if(temp.size() + 1 > bufferlen)
return outcome_e::errc::no_buffer_space;
// Copy the string into the supplied buffer, and return length of string
memcpy(buffer, temp.data(), temp.size() + 1);
return temp.size();
}
catch(...)
{
// This is the <system_error2> analogue of Standard Outcome's
// error_from_exception() utility function. It consumes an exception
// ptr (defaulted to current exception), and tries to match it to a
// standard C++ library exception type, returning a system_code
// with an appropriate code domain (generic_code, posix_code,
// win32_code).
//
// Note that using this function requires including
// <boost/outcome/experimental/system_code_from_exception.hpp>
// It is NOT included by Experimental Outcome by default.
return outcome_e::system_code_from_exception();
}
}
As the alias status_result<size_t> defaults the erased type to the alias system_code,
the to_string() function returns (in concrete types) basic_result<size_t, status_code<erased<intptr_t>>>.
The standard Outcome function referenced is documented at
std::error_code error_from_exception(std::exception_ptr &&ep = std::current_exception(), std::error_code not_matched = std::make_error_code(std::errc::resource_unavailable_try_again)) noexcept
.
The proposed <system_error2> reference library implementation provides an identically named
function taking similar parameters, but it returns a outcome_e::system_code (status_code<erased<intptr_t>>) instead of a std::error_code.