Generate string if boolean attribute is true (match karma with qi :: match)
Imagine we want to parse and generate a simple C ++ member function declaration using Boost.Spirit.
Qi grammar Qi might look like this:
function_ %= type_ > id_ > "()" > matches["const"];
This means that the function const
is stored in bool
.
How to write an appropriate generator with karma?
function_ %= type_ << ' ' << id_ << "()" << XXX[" const"];
Here we need a directive that consumes a boolean attribute, executes an inline generator if the attribute true
does nothing. We need something that makes the following tests successful.
test_generator_attr("abc", XXX["abc"], true);
test_generator_attr("", XXX["abc"], false);
Is such a directive already available in Boost.Spirit?
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The first thing that comes to my mind is
bool const_qualifier = true;
std::cout << karma::format(
karma::omit[ karma::bool_(true) ] << " const" | "",
const_qualifier);
Feels a little ... awkward. I'll see later what I forgot :)
UPDATE Here's a slightly more elegant usage with karma::symbols<>
:
#include <boost/spirit/include/karma.hpp>
namespace karma = boost::spirit::karma;
int main()
{
karma::symbols<bool, const char*> const_;
const_.add(true, "const")(false, "");
for (bool const_qualifier : { true, false })
{
std::cout << karma::format_delimited("void foo()" << const_, ' ', const_qualifier) << "\n";
}
}
Printing
void foo() const
void foo()
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