“转换”函数参数(“Transforming” Function Arguments)

 我正在编写与库接口的C ++类。 该库有许多函数，如下所示： 
 
 Library.h 
 
int FunctionA(int deviceNumber, ...);
int FunctionB(int deviceNumber, ...);
int FunctionC(int deviceNumber, ...);
int FunctionD(int deviceNumber, ...);
int FunctionE(int deviceNumber, ...);
 
 我的C ++类的每个实例都有一个永远不会改变的关联deviceNumber ，所以我将deviceNumber存储为成员变量，每次调用库函数时，我都将成员作为函数调用的第一个参数传递。 
 
 这没关系，我没有理由改变它的方式。 但出于好奇，我想知道C ++是否有任何“转换”参数的机制可以让我避免在每个调用中传递相同的参数。 实现这一目标的显而易见的方法是超载一切。 假设我的班级叫做Foo ： 
 
 Foo.cpp中 
 
int Foo::FunctionA(...) {
    // deviceNumber_ is a const member
    return ::FunctionA(deviceNumber_, ...);
}
 
 唯一的问题是这需要每个函数调用的方法，因此随着库的增长，没有代码生成器它会变得越来越烦人。 
 
 是否有任何通用的方法来提供重载行为而不实际重载函数？ C ++中是否有一种机制可以将参数“扩展”为多个参数？ 我想象它看起来像： 
 
// These two calls are equivalent
FunctionA(deviceNumber, ...);
FunctionA(Magic(...));
// Magic() expands the arguments, adding deviceNumber
 
 即使解决方案比单独留下一切更加丑陋和可读性，我很好奇是否可能。 搜索后，可变参数模板似乎是最接近的匹配，但我无法真正地了解如何使用它们来完成此任务。 

I'm writing C++ class that interfaces with a library. The library has a bunch of functions that look like the following:
 
Library.h
 
int FunctionA(int deviceNumber, ...);
int FunctionB(int deviceNumber, ...);
int FunctionC(int deviceNumber, ...);
int FunctionD(int deviceNumber, ...);
int FunctionE(int deviceNumber, ...);
 
Each instance of my C++ class has an associated deviceNumber which never changes, so I have deviceNumber stored as a member variable, and every time I call a library function, I pass the member in as the function call's first argument. 
 
This is fine, and there's no real reason for me to change the way it is. But out of curiosity, I was wondering if C++ had any mechanism to "transform" arguments that would let me avoid passing the same argument in every call. The obvious way to accomplish this is to overload everything. Let's say my class is called Foo:
 
Foo.cpp
 
int Foo::FunctionA(...) {
    // deviceNumber_ is a const member
    return ::FunctionA(deviceNumber_, ...);
}
 
The only problem is that this requires a method for each function call, so as the library grows, it gets more and more annoying without a code generator.
 
Is there any general way to provide the overloading behaviour without actually overloading the functions? Is there a mechanism in C++ to "expand" an argument into multiple arguments? I'm imagining it would look like:
 
// These two calls are equivalent
FunctionA(deviceNumber, ...);
FunctionA(Magic(...));
// Magic() expands the arguments, adding deviceNumber
 
Even if the solution is much uglier and less readable than leaving everything alone, I'm curious if it's possible. After searching around, variadic templates seem to be the closest match, but I can't really wrap my head around how they could be used to accomplish this. 

原文：https://stackoverflow.com/questions/28909656