Using find_if on a vector of object

Question

I have a vector of that looks like the following:

class Foo {     //whatever };  class MyClass {     int myInt;     vector<Foo> foo_v; }; 

And let's say, in the main:

int main (void) {     vector<MyClass> myClass_v; } 

I want to find a object in myClass_v that has myInt == bar. I don't care about foo_v. I thought of using the std::find_if function:

std::find_if(myClass_v.begin(),myClass_v.end(),condition); 

with

bool MyClass::condition(MyClass mc) {     if(mc.myInt==5)         return true;     else         return false; } 

However the compiler says that condition() is missing arguments. Could you tell me what am I doing wrong? I thought that std::find_if would call condition(*First), with First being a pointer to a myClass object.

Or is there another good way to do the same thing?

Answer 1

That's not how predicates work. You have to supply either a free function bool Comparator(const MyClass & m) { ... }, or build a function object, a class that overloads operator():

struct MyClassComp {   explicit MyClassComp(int i) n(i) { }   inline bool operator()(const MyClass & m) const { return m.myInt == n; } private:   int n; };  std::find_if(v.begin(), v.end(), MyClassComp(5)); 

In C++0x:

std::find_if(v.begin(), v.end(),              [](const MyClass & m) -> bool { return m.myInt == 5; }); 

This captureless lambda is in fact equivalent to a free function. Here is a capturing version that mimics the predicate object:

const int n = find_me(); std::find_if(v.begin(), v.end(),              [n](const MyClass & m) -> bool { return m.myInt == n; });