C++ :: Find Function To A Vector Of Structures Vector

I have asked a related question before, and it was resolved successfully. In the past, when I wanted to use std::max_element in order to find the maximum element (or even sort by using std::sort) of a vector of structures according to one of the members of the structure, all I had to do was to insert a specially designed comparison function as the third argument of the function std::max::element. But the latter comparison function naturally accepts two arguments internally.

For instance, here is a test program that successfully finds the maximum according to just one member of the structure:

Code:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

[Code] ....

And the output was this, as expected:
Maximum element S.a of vector<S> vec is at: 9
[I]max element of vec.a between slot 3 and slot 6 is: 6, and its index is: 6 vec[6].a = 6
[I]max element of vec.a between slot 4 and slot 7 is: 7, and its index is: 7 vec[7].a = 7
[I]max element of vec.a between slot 5 and slot 8 is: 8, and its index is: 8 vec[8].a = 8
[I]max element of vec.a between slot 6 and slot 9 is: 9, and its index is: 9 vec[9].a = 9

However, I now need to search and find an element of vector<myStruct> according to just one member of myStruct, instead of finding the maximum or sorting as before. This presents a problem because the function std::find does not accept such a comparison function as its third argument.

This was the description of the std::find function that I found: find - C++ Reference

Code:
template <class InputIterator, class T> InputIterator find (InputIterator first, InputIterator last, const T& val);

I could also find another function called std::find_if, but this only accepts a unary predicate like this: find_if - C++ Reference

Code:
template <class InputIterator, class UnaryPredicate> InputIterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred);

And once again this is either inadequate of I don't see how to use it directly, because for the third argument I would like to insert a function that takes two arguments with a syntax like this:

Code:
int x=7;
std::vector<S>::iterator result;
result = std::find(vec.begin(), vec.end(), []( const (int x, const S & S_1) { return ( x == S_1.a ) ; } ) ;

Is there another std function that I can use to make search and find an element according to just one member of myStruct?

Or perhaps there is a clever way to pass two arguments to the unary predicate.