C++ 将参数传递给比较函数？
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Passing a parameter to a comparison function?
提问by George41
When using the STL sort algorithm on a vector, I want to pass in my own comparison function which also takes a parameter.
在向量上使用 STL 排序算法时，我想传入我自己的比较函数，该函数也接受一个参数。
For example, ideally I want to do a local function declaration like:
例如，理想情况下我想做一个局部函数声明，如：
int main() {
vector<int> v(100);
// initialize v with some random values
int paramA = 4;
bool comp(int i, int j) {
// logic uses paramA in some way...
}
sort(v.begin(), v.end(), comp);
}
However, the compiler complains about that. When I try something like:
但是，编译器对此有所抱怨。当我尝试类似的事情时：
int main() {
vector<int> v(100);
// initialize v with some random values
int paramA = 4;
struct Local {
static bool Compare(int i, int j) {
// logic uses paramA in some way...
}
};
sort(v.begin(), v.end(), Local::Compare);
}
The compiler still complains: "error: use of parameter from containing function"
编译器仍然抱怨：“错误：使用包含函数的参数”
What should I do? Should I make some global variables with a global comparison function..?
我该怎么办？我应该使用全局比较函数创建一些全局变量吗？
Thanks.
谢谢。
回答by Adam Rosenfield
You cannot access the local variables of a function from within a locally defined function -- C++ in its current form does not allow closures. The next version of the language, C++0x, will support this, but the language standard has not been finalized and there is little support for the current draft standard at the moment.
您不能从局部定义的函数内部访问函数的局部变量——当前形式的 C++ 不允许闭包。该语言的下一版本 C++0x 将支持这一点，但语言标准尚未最终确定，目前对当前草案标准的支持很少。
To make this work, you should change the third parameter of std::sortto be an object instanceinstead of a function. The third parameter of std::sortcan be anything that is callable (i.e. any xwhere adding parentheses like x(y, z)makes syntactic sense). The best way to do this is to define a struct that implements the operator()function, and then pass an instance of that object:
为了使这项工作，你应该改变的第三个参数std::sort是一个对象实例，而不是一个功能。的第三个参数std::sort可以是任何可调用的参数（即x，在x(y, z)句法上添加括号的任何地方）。最好的方法是定义一个实现该operator()函数的结构体，然后传递该对象的一个实例：
struct Local {
Local(int paramA) { this->paramA = paramA; }
bool operator () (int i, int j) { ... }
int paramA;
};
sort(v.begin(), v.end(), Local(paramA));
Note that we have to store paramAin the structure, since we can't access it otherwise from within operator().
请注意，我们必须存储paramA在结构中，因为否则我们无法从 内部访问它operator()。
回答by Prasoon Saurav
In C++ you cannot define a free function inside another function. So your first code snippet is ill formed.
在 C++ 中，您不能在另一个函数中定义自由函数。所以你的第一个代码片段格式不正确。
sort(v.begin(), v.end(), Local::Compare);
sort(v.begin(), v.end(), Local::Compare);
The 3rd argument must be a function object. Overload ()operator inside the class and then create the function object.
第三个参数必须是一个函数对象。()在类内部重载运算符，然后创建函数对象。
In C++0x you can use lambda expressions.
在 C++0x 中，您可以使用lambda 表达式。
auto comp = [&](int m,int n)-> bool {
return m<n; //or use paramA in some way
};
sort(v.begin(), v.end(), comp);
回答by Jerry Coffin
One possibility is to pass the parameter when you construct your comparator object:
一种可能性是在构造比较器对象时传递参数：
class cmp {
int param;
public:
cmp(int p) : param(p) {}
bool operator()(int i, int j) {
// logic uses param
}
};
int main() {
vector<int> v(100);
// initialize v with some random values
int paramA = 4;
sort(v.begin(), v.end(), cmp(paramA));
}
回答by dang son
//Using std::bind
//使用 std::bind
//Example
//例子
{
vector<int> vecInt{2, 4, 10, 20, 30};
int i = 4;
sort(vecInt.begin(), vecInt.end(), std::bind( [](int a, int b, int c)
{
return abs(a - c) < abs(b - c);
}, std::placeholders::_1, std::placeholders::_2, i)
);
}
