function object, function pointer and function.

function object, function pointer and function.

[learning c++]

Hi, Everyone:

I compiled a short code that invovles function object and function pointer.

in the line:  int count1 = count_if(v1.begin(), v1.end(), 
&lessThan20Function);

I used the funtion poiter to pass the address of function. It works very 
well,

However, when I deleted the symbol "&", it still works. "lessThan20Function" 
is the name of function. Can we use the function name as an argument?

What is the advantage of function object?

Thanks in advanace!

#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

bool lessThan20Function(int value)
{
    return value < 20;
}

class LessThan20 {
public:
    bool operator() (int value) const
    {
        return value < 20;
    }
};

int main() {

    int iarr[10] = {2,4,55,6,66,10,22,14,70,1234};
    vector<int> v1(iarr, iarr+10);

    LessThan20 lt20;
    // count_if(start_iterator, end_iterator, predicate);

    // Pointer to a function as argument
    int count1 = count_if(v1.begin(), v1.end(), &lessThan20Function);
    cout << "There are " << count1 << " values less than 20" << endl;

    // Function object as argument
    int count2 = count_if(v1.begin(), v1.end(), lt20);
    cout << "There are " << count2 << " values less than 20" << endl;
    return 0;
]

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Re: function object, function pointer and function.

[Eljay Love-Jensen]

Hi,

In the situation you presented, the & in your "int count1 = 
count_if(v1.begin(), v1.end(), &lessThan20Function);" line is 
superfluous.  Although it is perfectly good code, and acceptable to all 
compilers.

In my experience, it would be omitted.  By convention.

Also in my experience, there is a preference to function objects over 
function pointers.

For some really neat stuff, the BOOST (www.boost.org) library has a lambda 
library, which would allow you to put the body of the predicate right in 
your count_if statement -- and in a quite abbreviated form.  It'd look 
something like...

int count = count_if(v1.begin(), v1.end(), _1 > 20);

HTH,
--Eljay