MRDOCS_TRY_BIND

Evaluate an expected‐like expression into a structured binding.

Synopsis

Declared in <mrdocs/Support/Error/Expected.hpp>

#define MRDOCS_TRY_BIND(names, expr)

Description

Works like the two‐argument form of MRDOCS_TRY, but unpacks the value into a structured binding. The expression is evaluated once. If it failed, the enclosing function returns Unexpected with its error. Otherwise the value is moved into auto [names...]. The enclosing function must return an mrdocs::Expected. See MRDOCS_TRY for what counts as failed.

Wrap the binding names in parentheses, so the commas between them aren't taken as macro‐argument separators:

Expected<std::pair<std::string, int>>
splitKeyValue(std::string_view line);

Expected<Option>
parseOption(std::string_view line)
{
    MRDOCS_TRY_BIND((key, value), splitKeyValue(line));
    return Option{std::move(key), value};
}

The body of parseOption expands to roughly this:

// Simplified: the hidden local is really named
// expected_result_<line>, and failed/error are
// ::mrdocs::detail::failed and ::mrdocs::detail::error.

// MRDOCS_TRY_BIND((key, value), splitKeyValue(line));
auto tmp = splitKeyValue(line);
if (failed(tmp)) {
    return Unexpected(error(tmp));
}
auto [key, value] = *std::move(tmp);

return Option{std::move(key), value};

Any type that supports structured bindings works, with as many names as it has elements:

Expected<std::tuple<std::string, int, int>>
parseLocation(std::string_view text);

Expected<int>
lineOf(std::string_view text)
{
    MRDOCS_TRY_BIND((file, line, column), parseLocation(text));
    return line;
}

Things to keep in mind:

The binding is always auto [...]: you can't make it const or a reference. The names bind to a fresh object moved out of the result.
There's no custom‐message form. If you need one, use MRDOCS_TRY(auto v, expr, msg) and bind v yourself.
Like MRDOCS_TRY, it expands to several statements, needs a trailing semicolon, can't appear twice on the same line, and needs braces when it's the body of an if, else, or loop.

Parameters

Name

Description

names

The parenthesized list of binding names.

expr

The expected‐like expression to evaluate.

Created with MrDocs