Following the request in comments, here is how to implement an arithmetic parser using boost::spirit. First, you need to download the boost tarball, don't try to just clone Spirit alone from GitHub, because it has dependencies from other boost libraries.
Boost is huge, so if you want just a subset enough for a parser, you can extract it using bcp. From boost source directory:
cd tools/build/src/engine
./build.sh
cd ../../../bcp
../build/src/engine/b2
cd ../..
dist/bin/bcp fusion/include hana/functional spirit/home/x3 /some/path
bcp will copy all dependencies. You can leave only /some/path/boost directory, because all libraries we need are header only.
Finally, here is the full code of the parser.
#include <iostream>
#include <numeric>
#include <stdexcept>
#include <string>
#include <vector>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/hana/functional/fix.hpp>
#include <boost/hana/functional/overload.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/support/ast/variant.hpp>
using namespace boost::spirit;
namespace hana = boost::hana;
// Define AST. The root is `ast::expr`, which is the first left-hand side
// operand and a list of all operations on the right-hand side. Each operand is
// a recursive `variant` that has `ast::expr` inside.
namespace ast
{
struct nil {};
struct signed_;
struct expr;
struct operand : x3::variant<
nil
, double
, x3::forward_ast<signed_>
, x3::forward_ast<expr>
>
{
using base_type::base_type;
using base_type::operator=;
};
struct signed_
{
char sign;
operand operand_;
};
struct operation
{
char operator_;
operand operand_;
};
struct expr
{
operand first;
std::vector<operation> rest;
};
} // namespace ast
// Give the grammar access to the fields of AST.
BOOST_FUSION_ADAPT_STRUCT(ast::signed_, sign, operand_)
BOOST_FUSION_ADAPT_STRUCT(ast::operation, operator_, operand_)
BOOST_FUSION_ADAPT_STRUCT(ast::expr, first, rest)
// Arithmetic expression grammar definition.
namespace ArithExpr
{
x3::rule<class expression, ast::expr > const expression("expression");
x3::rule<class term, ast::expr > const term("term");
x3::rule<class factor, ast::operand> const factor("factor");
auto const expression_def =
term
>> *(
(x3::char_('+') >> term)
| (x3::char_('-') >> term)
);
auto const term_def =
factor
>> *(
(x3::char_('*') >> factor)
| (x3::char_('/') >> factor)
);
auto const factor_def =
x3::double_
| '(' >> expression >> ')'
| (x3::char_('-') >> factor)
| (x3::char_('+') >> factor);
BOOST_SPIRIT_DEFINE(expression, term, factor);
auto calc = expression;
} // namespace ArithExpr
template <typename Iterator>
double CalcArithExpr(Iterator const &first, Iterator last) {
ast::expr expr;
// Build AST.
if (!x3::phrase_parse(first, last, ArithExpr::calc, x3::ascii::space, expr)) {
throw std::runtime_error("Cannot parse arithmetic expression");
}
// Parse the AST and calculate the result.
// hana::fix allows recursive lambda call
auto astEval = hana::fix([](auto self, auto expr) -> double {
// hana::overload calls a lambda corresponding to the type in the variant
return hana::overload(
[](ast::nil) -> double {
BOOST_ASSERT(0);
return 0;
},
[](double x) -> double { return x; },
[&](ast::signed_ const &x) -> double {
double rhs = boost::apply_visitor(self, x.operand_);
switch (x.sign) {
case '-': return -rhs;
case '+': return +rhs;
}
BOOST_ASSERT(0);
return 0;
},
[&](ast::expr const &x) -> double {
return std::accumulate(
x.rest.begin(), x.rest.end(),
// evaluate recursively left-hand side
boost::apply_visitor(self, x.first),
[&](double lhs, const ast::operation &op) -> double {
// evaluate recursively right-hand side
double rhs = boost::apply_visitor(self, op.operand_);
switch (op.operator_) {
case '+': return lhs + rhs;
case '-': return lhs - rhs;
case '*': return lhs * rhs;
case '/': return lhs / rhs;
}
BOOST_ASSERT(0);
return 0;
}
);
}
)(expr);
});
return astEval(expr);
}
int main(int argc, char *argv[]) {
auto expr = std::string{"-(4.5 + 5e-1) * 2.22 - 9.1 / 3.45"};
std::cout << CalcArithExpr(expr.begin(), expr.end()) << std::endl;
}
It calculates -(4.5 + 5e-1) * 2.22 - 9.1 / 3.45 and outputs -13.7377.
Update
Here are instructions how to build bcp and copy selected headers on Windows. Though, without any guarantee. In Linux everything just works, on Windows it is always jumps over some hoops, and the direction of jumps are always unpredictable.
This being said, open PowerShell command line. There
Import-Module 'C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\Common7\Tools\Microsoft.VisualStudio.DevShell.dll'
Install-Module VSSetup -Scope CurrentUser
Get-VSSetupInstance
Substitute 2019 above with your version of VS. You have to do it only once for your PowerShell. The rest is every time you need to build bcp. Get-VSSetupInstance above will print information about the instances of Visual Studio you have on your machine. Write down InstanceId that you would like to use. Now change to the boost directory in the PowerShell, and:
Enter-VsDevShell InstanceId -DevCmdArguments '-arch=x64' -SkipAutomaticLocation
Where InstanceId is the ID you got from Get-VSSetupInstance. Then from the same command prompt
cd tools\build\src\engine
& .\build.bat
cd ..\..\..\bcp
..\build\src\engine\b2 address-model=64
cd ..\..
dist\bin\bcp fusion\include hana\functional spirit\home\x3 X:\some\path\boost
printf("Result: %19.15\n",result);should beprintf("Result: %19.15f\n",result);– Sigismund