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; });