C++ 按对象属性搜索对象向量
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Search a vector of objects by object attribute
提问by Christoffer
I'm trying to figure out a nice way to find the index of a certain object in a vector - by comparing a string to a member field in the object.
我试图找出一种很好的方法来查找向量中某个对象的索引 - 通过将字符串与对象中的成员字段进行比较。
Like this:
像这样：
find(vector.begin(), vector.end(), [object where obj.getName() == myString])
I have searched without success - maybe I don't fully understand what to look for.
我搜索没有成功 - 也许我不完全明白要寻找什么。
回答by juanchopanza
You can use std::find_ifwith a suitable functor. In this example, a C++11 lambda is used:
您可以使用std::find_if合适的函子。在此示例中，使用了 C++11 lambda：
std::vector<Type> v = ....;
std::string myString = ....;
auto it = find_if(v.begin(), v.end(), [&myString](const Type& obj) {return obj.getName() == myString;})
if (it != v.end())
{
// found element. it is an iterator to the first matching element.
// if you really need the index, you can also get it:
auto index = std::distance(v.begin(), it);
}
If you have no C++11 lambda support, a functor would work:
如果您没有 C++11 lambda 支持，函子可以工作：
struct MatchString
{
MatchString(const std::string& s) : s_(s) {}
bool operator()(const Type& obj) const
{
return obj.getName() == s_;
}
private:
const std::string& s_;
};
Here, MatchStringis a type whose instances are callable with a single Typeobject, and return a boolean. For example,
这里，MatchString是一个类型，它的实例可以用单个Type对象调用，并返回一个布尔值。例如，
Type t("Foo"); // assume this means t.getName() is "Foo"
MatchString m("Foo");
bool b = m(t); // b is true
then you can pass an instance to std::find
然后你可以将一个实例传递给 std::find
std::vector<Type>::iterator it = find_if(v.begin(), v.end(), MatchString(myString));
回答by Arne Mertz
In addition to the Lambda and the handwritten functor used by juancho, you have the possibility to use boost::bind(C++03) or std::bind(C++11) and a simple function:
除了 juancho 使用的 Lambda 和手写函子之外，您还可以使用boost::bind(C++03) 或std::bind(C++11) 和一个简单的函数：
bool isNameOfObj(const std::string& s, const Type& obj)
{ return obj.getName() == s; }
//...
std::vector<Type>::iterator it = find_if(v.begin(), v.end(),
boost::bind(&isNameOfObj, myString, boost::placeholders::_1));
Or, if Typehas a method isName:
或者，如果Type有一个方法isName：
std::vector<Type>::iterator it = find_if(v.begin(), v.end(),
boost::bind(&Type::isName, boost::placeholders::_1, myString));
This is just for completeness. In C++11 I'd prefer Lambdas, in C++03 I'd use bind only if the comparison function itself exists already. If not, prefer the functor.
这只是为了完整性。在 C++11 中，我更喜欢 Lambdas，在 C++03 中，我只在比较函数本身已经存在时才使用 bind。如果没有，则更喜欢函子。
PS:Since C++11 has no polymorphic/templated lambdas, bind still has it's place in C++11, e.g. if the parameter types are unknown, hard to spell, or otherwise not easy to deduce.
PS：由于 C++11 没有多态/模板化的 lambdas，bind 在 C++11 中仍然占有一席之地，例如，如果参数类型未知、难以拼写或不容易推断。
回答by uniqrish
A simple iterator may help.
一个简单的迭代器可能会有所帮助。
typedef std::vector<MyDataType> MyDataTypeList;
// MyDataType findIt should have been defined and assigned
MyDataTypeList m_MyObjects;
//By this time, the push_back() calls should have happened
MyDataTypeList::iterator itr = m_MyObjects.begin();
while (itr != m_MyObjects.end())
{
if(m_MyObjects[*itr] == findIt) // any other comparator you may want to use
// do what ever you like
}
