C++ 如何用 lambda 排序？
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How to sort with a lambda?
提问by BTR
sort(mMyClassVector.begin(), mMyClassVector.end(),
[](const MyClass & a, const MyClass & b)
{
return a.mProperty > b.mProperty;
});
I'd like to use a lambda function to sort custom classes in place of binding an instance method. However, the code above yields the error:
我想使用 lambda 函数对自定义类进行排序，而不是绑定实例方法。但是，上面的代码产生了错误：
error C2564: 'const char *' : a function-style conversion to a built-in type can only take one argument
错误 C2564：“const char *”：到内置类型的函数式转换只能采用一个参数
It works fine with boost::bind(&MyApp::myMethod, this, _1, _2).
它与boost::bind(&MyApp::myMethod, this, _1, _2).
回答by BTR
Got it.
知道了。
sort(mMyClassVector.begin(), mMyClassVector.end(),
[](const MyClass & a, const MyClass & b) -> bool
{
return a.mProperty > b.mProperty;
});
I assumed it'd figure out that the > operator returned a bool (per documentation). But apparently it is not so.
我认为它会发现 > 运算符返回了一个布尔值（每个文档）。但显然并非如此。
回答by Adrian
To much code, you can use it like this:
对于很多代码，您可以像这样使用它：
#include<array>
#include<functional>
int main()
{
std::array<int, 10> vec = { 1,2,3,4,5,6,7,8,9 };
std::sort(std::begin(vec ), std::end(vec ), [](int a, int b) {return a > b; });
for (auto item : vec)
std::cout << item << " ";
return 0;
}
Replace "vec" with your class and that's it.
用你的班级替换“vec”，就是这样。
回答by Stephan
Can the problem be with the "a.mProperty > b.mProperty" line? I've gotten the following code to work:
问题可能出在“a.mProperty > b.mProperty”行上吗？我已经得到以下代码工作：
#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>
#include <sstream>
struct Foo
{
Foo() : _i(0) {};
int _i;
friend std::ostream& operator<<(std::ostream& os, const Foo& f)
{
os << f._i;
return os;
};
};
typedef std::vector<Foo> VectorT;
std::string toString(const VectorT& v)
{
std::stringstream ss;
std::copy(v.begin(), v.end(), std::ostream_iterator<Foo>(ss, ", "));
return ss.str();
};
int main()
{
VectorT v(10);
std::for_each(v.begin(), v.end(),
[](Foo& f)
{
f._i = rand() % 100;
});
std::cout << "before sort: " << toString(v) << "\n";
sort(v.begin(), v.end(),
[](const Foo& a, const Foo& b)
{
return a._i > b._i;
});
std::cout << "after sort: " << toString(v) << "\n";
return 1;
};
The output is:
输出是：
before sort: 83, 86, 77, 15, 93, 35, 86, 92, 49, 21,
after sort: 93, 92, 86, 86, 83, 77, 49, 35, 21, 15,
