结果我在不依赖 boost 的情况下做了一个编译时的 S 表达式。唯一的问题是依赖于 c++11
性别公关
#ifndef S_EXPRESSION_H
#define S_EXPRESSION_H
#include <algorithm>
template< typename... Subexprs > class SExpr;
template< typename SExprType, unsigned int Idx > class SExprGetter;
template< typename SExprType >
class SExprGetter< SExprType, 0 >
{
SExprType* tree;
public:
typedef typename SExprType::Expr_0_type return_type;
SExprGetter(SExprType& _v) : tree(& _v) {}
SExprGetter(SExprGetter&& _o) : tree(_o.tree) {}
return_type& getValue() { return tree->get_storage(); }
template< unsigned int Idx>
SExprGetter< return_type , Idx > getChild()
{
return getValue().template getChild<Idx>();
}
static constexpr unsigned int childs()
{
return return_type::childs();
}
};
template< typename SExprType >
class SExprGetter< SExprType, 1 >
{
SExprType* tree;
typedef typename SExprType::Remainder_type remainder_type_1;
public:
typedef typename remainder_type_1::Expr_0_type return_type;
SExprGetter(SExprType& _v) : tree(& _v) {}
SExprGetter(SExprGetter&& _o) : tree(_o.tree) {}
return_type& getValue() { return tree->get_remainder_storage().get_storage(); }
template< unsigned int Idx>
SExprGetter< return_type , Idx > getChild()
{
return getValue().template getChild<Idx>();
}
static constexpr unsigned int childs()
{
return return_type::childs();
}
};
template< typename SExprType >
class SExprGetter< SExprType, 2 >
{
SExprType* tree;
typedef typename SExprType::Remainder_type remainder_type_1;
typedef typename remainder_type_1::Remainder_type remainder_type_2;
public:
typedef typename remainder_type_2::Expr_0_type return_type;
SExprGetter(SExprType& _v) : tree(& _v) {}
SExprGetter(SExprGetter&& _o) : tree(_o.tree) {}
return_type& getValue() { return tree->get_remainder_storage().get_remainder_storage().get_storage(); }
template< unsigned int Idx>
SExprGetter< return_type , Idx > getChild()
{
return getValue().template getChild<Idx>();
}
static constexpr unsigned int childs()
{
return return_type::childs();
}
};
template< typename SExprType >
class SExprGetter< SExprType, 3 >
{
SExprType* tree;
typedef typename SExprType::Remainder_type remainder_type_1;
typedef typename remainder_type_1::Remainder_type remainder_type_2;
typedef typename remainder_type_2::Remainder_type remainder_type_3;
public:
typedef typename remainder_type_3::Expr_0_type return_type;
SExprGetter(SExprType& _v) : tree(& _v) {}
SExprGetter(SExprGetter&& _o) : tree(_o.tree) {}
return_type& getValue() { return tree->get_remainder_storage().get_remainder_storage().get_remainder_storage().get_storage(); }
template< unsigned int Idx>
SExprGetter< return_type , Idx > getChild()
{
return getValue().template getChild<Idx>();
}
static constexpr unsigned int childs()
{
return return_type::childs();
}
};
template< typename T0, typename... Subexprs >
class SExpr< T0, Subexprs... >
{
T0 _t0_storage;
SExpr< Subexprs... > _remainder_storage;
public:
typedef T0 Expr_0_type;
typedef SExpr< Subexprs... > Remainder_type;
SExpr(T0 _t, Subexprs... sexprs) : _t0_storage(_t), _remainder_storage(sexprs...) {}
SExpr(T0&& _t, Subexprs && ... sexprs) : _t0_storage(_t), _remainder_storage(sexprs...) {}
T0& get_storage() { return _t0_storage; }
Remainder_type& get_remainder_storage() { return _remainder_storage; }
template< unsigned int Idx>
SExprGetter< SExpr , Idx > getChild()
{
SExprGetter< SExpr, Idx > getter(*this);
return getter;
}
static constexpr unsigned int childs()
{
return sizeof...(Subexprs) + 1;
}
};
template< typename T0>
class SExpr< T0 >
{
T0 _t0_storage;
public:
typedef T0 Expr_0_type;
typedef void Remainder_type;
SExpr(T0 _t) : _t0_storage(_t) {}
SExpr(T0&& _t) : _t0_storage(_t) {}
T0& get_storage() { return _t0_storage; }
static constexpr unsigned int childs()
{
return 1;
}
template< unsigned int Idx>
SExprGetter< SExpr , Idx > getChild()
{
SExprGetter< SExpr, Idx > getter(*this);
return getter;
}
};
template <typename... Exprs>
SExpr<Exprs...> _S_expr(Exprs... exprs) {
return
{
exprs...
};
};
#endif /* S_EXPRESSION_H */
测试.cpp
#include <iostream>
#include "sexpr.h"
#include <string>
using namespace std;
int main()
{
auto s_expr_1 = _S_expr(3);
auto s_expr_2 = _S_expr(3, std::string("foo"));
auto s_expr_3 = _S_expr(3, _S_expr(3, "bar"), std::string("foo"));
auto ff_1 = s_expr_1.getChild<0>().getValue();
cout << " ff_1: " << ff_1 << endl;
auto ff_2 = s_expr_2.getChild<1>().getValue();
cout << " ff_2: " << ff_2 << endl;
auto ff_3 = s_expr_3.getChild<1>().getChild<1>().getValue();
cout << " ff_3: " << ff_3 << endl;
cout << " s expr 3 has " << s_expr_3.childs() << " and the first leaf has " << s_expr_3.getChild< s_expr_3.childs() - 2 >().childs() << endl;
};