问题描述
我在看下面的问题：
我得到的格式如下字符串：
I get strings that are formatted like this:
functionname_parameter1_parameter2_parameter3
otherfunctionname_parameter1_parameter2
.
.
.
和我想调用该函数具有给定参数。
所以我们可以说我有一个功能测试：
and i would like to call the function with the given parameters.So let's say i have a function test:
void test(int x, float y, std::string z) {}
和我得到的消息：
test_5_2.0_abc
然后我想功能测试像这样被自动调用：
then i would like the function test to be automatically invoked like this:
test(5, 2.0, "abc");
你对如何用C实现这一点任何提示++？
Do you have any hints on how to accomplish this in C++?
推荐答案
更新：更新 stream_function 修复参数评估顺序问题@Nawaz在评论中提到，并且也去掉了的std ::功能以提高效率。 注意，评价阶修复仅适用于铛，因为GCC不遵循标准此处的GCC的一个例子，具有手动顺序执行，可以发现的。
Update: Updated stream_function to fix the argument-evaluation-order problem @Nawaz mentioned in the comments, and also removed the std::function for improved efficiency. An example for GCC, with manual order-enforcement, can be found here.
这通常不是那么容易实现。我写了周围的std ::功能有点包装类一旦从的std :: istream的提取参数。下面是使用一个例子C ++ 11：
This is generally not that easy to accomplish. I wrote a little wrapper class around std::function once that extracts the arguments from a std::istream. Here's an example using C++11:
#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <functional>
#include <stdexcept>
#include <type_traits>
// for proper evaluation of the stream extraction to the arguments
template<class R>
struct invoker{
R result;
template<class F, class... Args>
invoker(F&& f, Args&&... args)
: result(f(std::forward<Args>(args)...)) {}
};
template<>
struct invoker<void>{
template<class F, class... Args>
invoker(F&& f, Args&&... args)
{ f(std::forward<Args>(args)...); }
};
template<class F, class Sig>
struct stream_function_;
template<class F, class R, class... Args>
struct stream_function_<F, R(Args...)>{
stream_function_(F f)
: _f(f) {}
void operator()(std::istream& args, std::string* out_opt) const{
call(args, out_opt, std::is_void<R>());
}
private:
template<class T>
static T get(std::istream& args){
T t; // must be default constructible
if(!(args >> t)){
args.clear();
throw std::invalid_argument("invalid argument to stream_function");
}
return t;
}
// void return
void call(std::istream& args, std::string*, std::true_type) const{
invoker<void>{_f, get<Args>(args)...};
}
// non-void return
void call(std::istream& args, std::string* out_opt, std::false_type) const{
if(!out_opt) // no return wanted, redirect
return call(args, nullptr, std::true_type());
std::stringstream conv;
if(!(conv << invoker<R>{_f, get<Args>(args)...}.result))
throw std::runtime_error("bad return in stream_function");
*out_opt = conv.str();
}
F _f;
};
template<class Sig, class F>
stream_function_<F, Sig> stream_function(F f){ return {f}; }
typedef std::function<void(std::istream&, std::string*)> func_type;
typedef std::map<std::string, func_type> dict_type;
void print(){
std::cout << "print()\n";
}
int add(int a, int b){
return a + b;
}
int sub(int a, int b){
return a - b;
}
int main(){
dict_type func_dict;
func_dict["print"] = stream_function<void()>(print);
func_dict["add"] = stream_function<int(int,int)>(add);
func_dict["sub"] = stream_function<int(int,int)>(sub);
for(;;){
std::cout << "Which function should be called?\n";
std::string tmp;
std::cin >> tmp;
auto it = func_dict.find(tmp);
if(it == func_dict.end()){
std::cout << "Invalid function '" << tmp << "'\n";
continue;
}
tmp.clear();
try{
it->second(std::cin, &tmp);
}catch(std::exception const& e){
std::cout << "Error: '" << e.what() << "'\n";
std::cin.ignore();
continue;
}
std::cout << "Result: " << (tmp.empty()? "none" : tmp) << '\n';
}
}
锵3.3下的编译和按预期工作（）。
Which function should be called?
a
Invalid function 'a'
Which function should be called?
add
2
d
Error: 'invalid argument to stream_function'
Which function should be called?
add
2
3
Result: 5
Which function should be called?
add 2 6
Result: 8
Which function should be called?
add 2
6
Result: 8
Which function should be called?
sub 8 2
Result: 6
这很有趣，又劈了一起上课，希望你喜欢。请注意，您需要修改code稍微为您的示例工作，因为C ++输入输出流有空格分隔符为，所以你需要用空格替换所有下划线在您的邮件。应该很容易做到，虽然，在这之后仅仅建造一个的std :: istringstream 从消息：
std::istringstream input(message_without_underscores);
// call and pass 'input'
这篇关于带参数调用函数从字符串中提取的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！