如上所述,如果C++0x不可用,则Boost.Preprocessor是正确的选择,尽管需要一些时间来适应其语法。下面的示例演示了如何使用Boost.Preprocessor来定义具有可变(但有限)参数数量的函数。
#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/iteration/local.hpp>
#include <boost/preprocessor/iteration/iterate.hpp>
#define MAX_PARAMS 2
class sql {
public:
#define SQL_QUERY_DEF(z, n, unused) \
template <BOOST_PP_ENUM_PARAMS(n, class T)> \
void query(const std::string& query, \
BOOST_PP_ENUM_BINARY_PARAMS(n, const T, & x) );
#define BOOST_PP_LOCAL_MACRO(n) SQL_QUERY_DEF(~, n, ~)
#define BOOST_PP_LOCAL_LIMITS (1, MAX_PARAMS)
#include BOOST_PP_LOCAL_ITERATE()
...
};
#define SQL_VECTOR_CLEAR(z,i,var) var##i.clear();
#define SQL_VECTOR_PUSH_BACK(z,i,var) var##i.push_back(this->get_col<T##i>(i));
#define SQL_QUERY(z, n, unused) \
template <BOOST_PP_ENUM_PARAMS(n, class T)> \
void sql::query(const std::string& query, \
BOOST_PP_ENUM_BINARY_PARAMS(n, std::vector< T,>& x) ){ \
this->do_query(query); \
if(this->num_cols()<n){ \
throw std::runtime_error(); \
} \
BOOST_PP_REPEAT(n, SQL_VECTOR_CLEAR, x) \
while(this->is_open()) { \
BOOST_PP_REPEAT(n, SQL_VECTOR_PUSH_BACK, x) \
this->step(); \
} \
}
#define BOOST_PP_LOCAL_MACRO(n) SQL_QUERY(~, n, ~)
#define BOOST_PP_LOCAL_LIMITS (1, MAX_PARAMS)
#include BOOST_PP_LOCAL_ITERATE()
预处理器将此扩展为:
$ g++ -P -E sql.cpp | astyle
class sql {
public:
template < class T0> void query(const std::string& query, const T0 & x0 );
template < class T0 , class T1> void query(const std::string& query, const T0 & x0 , const T1 & x1 );
...
};
template < class T0> void sql::query(const std::string& query, std::vector< T0 >& x0 ) {
this->do_query(query);
if(this->num_cols()<1) {
throw std::runtime_error();
}
x0.clear();
while(this->is_open()) {
x0.push_back(this->get_col<T0>(0));
this->step();
}
}
template < class T0 , class T1> void sql::query(const std::string& query, std::vector< T0 >& x0 , std::vector< T1 >& x1 ) {
this->do_query(query);
if(this->num_cols()<2) {
throw std::runtime_error();
}
x0.clear();
x1.clear();
while(this->is_open()) {
x0.push_back(this->get_col<T0>(0));
x1.push_back(this->get_col<T1>(1));
this->step();
}
}
注意,这里我们不能使用BOOST_PP_REPEAT(MAX_PARAMS, SQL_QUERY, ~)
,因为它从0个参数开始复制,但我们需要从1个参数开始,这就是为什么需要BOOST_PP_LOCAL_ITERATE()
,它更加灵活。