Standard Template Library Programmer's Guide

many different function object adaptors, including unary_negate (which returns the logical complement of the value returned by a particular AdaptablePredicate), and unary_compose and binary_compose, which perform composition of function object.

Finally, the STL includes many different predefined function objects, including arithmetic operations (plus, minus, multiplies, divides, modulus, and negate), comparisons (equal_to, not_equal_to, greater, less, greater_equal, and less_equal), and logical operations (logical_and, logical_or, and logical_not). It is possible to perform very sophisticated operations without actually writing a new function object, simply by combining predefined function objects and function object adaptors.

Examples

Fill a vector with random numbers. In this example, the function object is simply a function pointer.

vector<int> V(100);
generate(V.begin(), V.end(), rand);

Sort a vector of double by magnitude, i.e. ignoring the elements' signs. In this example, the function object is an object of a user-defined class.

struct less_mag : public binary_function<double, double, bool> {
 bool operator()(double x, double y) { return fabs(x) < fabs(y); }
};
vector<double> V;
…
sort(V.begin(), V.end(), less_mag());

Find the sum of elements in a vector. In this example, the function object is of a user-defined class that has local state.

struct adder : public unary_function<double, void> {
 adder() : sum(0) {}
 double sum;
 void operator()(double x) { sum += x; }
};
vector<double> V;
…
adder result = for_each(V.begin(), V.end(), adder()); [3]
cout << 'The sum is ' << result.sum << endl;

Remove all elements from a list that are greater than 100 and less than 1000.

list<int> L;
…
list<int>::iterator new_end = remove_if(L.begin(), L.end(), compose2 (logical_and<bool>(), bind2nd(greater<int>(), 100), bind2nd(less<int>(), 1000)));
L.erase(new_end, L.end());

Concepts

• Generator

• Unary Function

• Binary Function

• Predicate

• Binary Predicate

• Adaptable Generator

• Adaptable Unary Function

• Adaptable Binary Function

• Adaptable Predicate

• Adaptable Binary Predicate

Types

• plus
• minus

• multiplies (formerly called times )

• divides
• modulus
• negate
• equal_to
• not_equal_to
• greater
• less
• greater_equal
• less_equal
• logical_and
• logical_or
• logical_not
• subtractive_rng
• identity
• project1st
• project2nd
• select1st
• select2nd
• unary_function
• binary_function
• unary_compose
• binary_compose
• unary_negate
• binary_negate
• binder1st
• binder2nd
• pointer_to_unary_function
• pointer_to_binary_function

Functions

• compose1
• compose2
• not1