Using lambdas with the STL

Using lambdas with the STL

Let;'s say you want to find the first instance of the variable named 'name' in one of our containers. The thing in the container is not a string, but a complex object with lots of fields, one of which happens to be a string that holds the name of the object. Here's the old, pre-c++-11 way using a function that returns a boolean and some adapters like bind2nd() and ptr_fun().

Change code like:

 // Note that in order for this to work the second argument must not be a reference.
 // jhrg 8/20/13
 static bool
 name_eq(D4Group *g, const string name)
 {
	return g->name() == name;
 }

 groupsIter g = find_if(grp_begin(), grp_end(), bind2nd(ptr_fun(name_eq), grp_name));

to:

 auto g = find_if(grp_begin(), grp_end(), [name](const D4Group *g) { return g->name() == name; });
                                           ^     ^                                       ^
                                           1     2                                       3

Where [name](const D4Group *g) { return g->name() == name; } is a C++ Lambda function (an anonymous function). This lambda function use uses a 'capture.' The square braces at #1 name the captured variable. Its value is taken from the current environment when the lambda is instantiated at runtime and used at #3 in the function. At #2 the argument to the lambda function is declared as a const pointer so the compiler knows the function won't be modifying the object. C++ STL functions like find_if() take predicates (which this lambda function is) and that can streamline code quite a bit.

The return value from find_if(...) is the iterator that references the first instance in the D4Group with a name that matches name.

Note that there a many algorithms in the STL that can perform operations like searching on all the elements of a container given the beginning and ending iterators.