C++ 非静态成员函数的无效使用
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invalid use of non-static member function
提问by Nash
I have something like this:
我有这样的事情：
class Bar
{
public:
pair<string,string> one;
std::vector<string> cars;
Bar(string one, string two, string car);
};
class Car
{
public:
string rz;
Bar* owner;
Car(string car, Bar* p);
};
class Foo
{
public:
Foo ( void );
~Foo ( void );
int Count ( const string & one, const string & two) const;
int comparator (const Bar & first, const Bar & second) const;
std::vector<Bar> bars;
};
int Foo::comparator(const Bar & first, const Bar & second) const{
return first.name < second.name;
}
int Foo::Count ( const string & one, const string & two ) const{
int result=0;
Bar mybar = Bar( one, two, "" );
std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);
if (ToFind != bars.end() && ToFind->one == mybar.one ){
result = ...
}
return result;
}
The method Foo::Countshould use std::lower_bound()to find element in vector<Bar>according to pair of two strings.
Now the part which doesn't work. To lower_bound()I'm providing method comparator(). I thought it was okay, but g++ says:
该方法Foo::Count应使用std::lower_bound()在找到元件vector<Bar>根据对的两个串。现在是不起作用的部分。要lower_bound()我提供方法comparator()。我认为没关系，但是 g++ 说：
c.cpp: In member function ‘int Foo::Count(const string&, const string&) const':
c.cpp:42:94: error: invalid use of non-static member function
std::vector<Bar>::iterator ToFind = lower_bound(bars.begin(), bars.end(), mybar, comparator);
And the method Count()must stay const...
方法Count()必须保持const......
I'm quite new to C++ because I'm forced to learn it.
我对 C++ 很陌生，因为我被迫学习它。
Any ideas?
有任何想法吗？
采纳答案by Matthew
You must make Foo::comparatorstatic or wrap it in a std::mem_funclass object. This is because lower_bounds()expects the comparer to be a class of object that has a call operator, like a function pointer or a functor object. Also, if you are using C++11 or later, you can also do as dwcanillassuggests and use a lambda function. C++11 also has std::bindtoo.
您必须将其Foo::comparator设为静态或将其包装在std::mem_fun类对象中。这是因为lower_bounds()期望比较器是具有调用运算符的对象类，如函数指针或函子对象。此外，如果您使用的是 C++11 或更高版本，您也可以按照dwcanillas 的建议使用 lambda 函数。C++11 也有std::bind。
Examples:
例子：
// Binding:
std::lower_bounds(first, last, value, std::bind(&Foo::comparitor, this, _1, _2));
// Lambda:
std::lower_bounds(first, last, value, [](const Bar & first, const Bar & second) { return ...; });
回答by M.M
The simplest fix is to make the comparator function be static:
最简单的解决方法是使比较器函数为静态：
static int comparator (const Bar & first, const Bar & second);
^^^^^^
When invoking it in Count, its name will be Foo::comparator.
在 中调用它时Count，它的名称将是Foo::comparator.
The way you have it now, it does not make sense to be a non-static member function because it does not use any member variables of Foo.
你现在拥有它的方式，成为一个非静态成员函数是没有意义的，因为它不使用Foo.
Another option is to make it a non-member function, especially if it makes sense that this comparator might be used by other code besides just Foo.
另一种选择是使它成为一个非成员函数，特别是如果这个比较器可能被除了Foo.
回答by edmz
You shall pass a thispointer to tell the function which object to work on because it relies on that as opposed to a staticmember function.
您应该传递一个this指针来告诉函数要处理哪个对象，因为它依赖于该对象而不是static成员函数。
