Mapbox variant C++11,C++14 library review

app1.cpp

#include <iostream>
#include <string> 
#include <sstream>
#include <cassert>

#include <mapbox/variant.hpp>

namespace m = mapbox::util;

// Forward declarations
struct Add;
struct Mul;

using expr = m::variant<int, m::recursive_wrapper<Add>, m::recursive_wrapper<Mul>>;

struct Add
{
    expr lhs;
    expr rhs;
    Add(expr lhs, expr rhs): lhs(lhs), rhs(rhs){ }
};

struct Mul
{
    expr lhs;
    expr rhs;
    Mul(expr lhs, expr rhs): lhs(lhs), rhs(rhs){ }
};

struct EvalVisitor
{
    int operator()(int x)
    {
        return x;
    }
    int operator()(Add const& v)
    {
        return m::apply_visitor(*this, v.lhs)
                + m::apply_visitor(*this, v.rhs);
    }

    int operator()(Mul const& v)
    {
        return m::apply_visitor(*this, v.lhs)
                * m::apply_visitor(*this, v.rhs);
    }

};


struct StringVisitor
{
    std::string format_expr(expr node)
    {
        if(node.is<int>())
            return std::to_string( node.get<int>() );
        else
            return std::string() + " (" + m::apply_visitor(*this, node) + ") ";
    }

    std::string operator()(int x)
    {
        return std::to_string(x);
    }
    std::string operator()(Add const& v)
    {
        return this->format_expr(v.rhs) + " + " + this->format_expr(v.lhs);
    }

    std::string operator()(Mul const& v)
    {
        return this->format_expr(v.rhs) + " * " + this->format_expr(v.lhs);
    }

};




int main(int argc, char** argv)
{
    std::cout << " [INFO] Running Ok " << std::endl;
    std::cout << std::boolalpha;

    // Abstract syntax tree
    expr ast;

    // Create visitor object
    auto eval = EvalVisitor{};
    auto disp = StringVisitor{};

    ast = 10;
    std::cout << " =>>  is_num? = " << ast.is<int>() << "\n";
    std::cout << " =>>    value = " << ast.get<int>() << "\n";
    std::cout << " =>>   Expr 0 = " << m::apply_visitor(disp, ast) << "\n";
    std::cout << " =>> Result 0 = " << m::apply_visitor(eval, ast) << '\n';

    ast = Mul(5, Add(3, 4));
    std::cout << "\n";
    std::cout << " =>>  is_num? = " << ast.is<int>() << "\n";
    std::cout << " =>>  is_add? = " << ast.is<Mul>() << "\n";
    std::cout << " =>>   Expr 1 = " << m::apply_visitor(disp, ast) << "\n";
    std::cout << " =>> Result 1 = " << m::apply_visitor(eval, ast) << '\n';

    // (10 + 4) * (5 * (3 + 8)) = 770
    ast        = Mul(Add(10, 4), Mul(5, Add(3, 8)));
    std::cout << "\n";
    std::cout << " =>>  is_mul? = " << ast.is<Mul>() << "\n";
    std::cout << " =>>   Expr 2 = " << m::apply_visitor(disp, ast) << "\n";
    int result = m::apply_visitor(eval, ast);
    assert(result == 770);
    std::cout << " =>> Result 2 = " << result  << '\n';


    return 0;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.9)
project(recursive-variant)

#========== Global Configurations =============#
#----------------------------------------------#

set(CMAKE_CXX_STANDARD 17)     
set(CMAKE_VERBOSE_MAKEFILE ON)
set(CMAKE_CXX_EXTENSIONS OFF)

# ------------ Download CPM CMake Script ----------------#

## Automatically donwload and use module CPM.cmake
file(DOWNLOAD https://raw.githubusercontent.com/TheLartians/CPM.cmake/v0.26.2/cmake/CPM.cmake
                 "${CMAKE_BINARY_DIR}/CPM.cmake")
include("${CMAKE_BINARY_DIR}/CPM.cmake")

#----------- Add dependencies --------------------------#

CPMAddPackage(
    NAME  mapbox-variant 
    URL   https://github.com/mapbox/variant/archive/v1.1.6.zip
    DOWNLOAD_ONLY YES
)

include_directories( ${mapbox-variant_SOURCE_DIR}/include )
message([TRACE] " mapbox source = ${mapbox-variant_SOURCE_DIR} ")
     
#----------- Set targets -------------------------------#

add_executable(app1 app1.cpp)
target_link_libraries(app1)