1. Definition
If the class defines the call operator (the function call operator is overloaded), the object of this class is called a function object, and the object of this class can be used like a function.
eg.
struct absInt{
int operator()(int val) const{
return val < 0 ? -val : val;
}
};
int i = -42;
absInt absObj;
int ui = absObj(i); //i被传递给absObj.operator()
2. Features
- The function call operator must be defined as a member function.
- . A class can define multiple different versions of the call operator, which must be different in the number or types of parameters.
- . Classes with function call operators can also store state, so they are more flexible than ordinary functions.
- . Function objects are often used as actual parameters of generic algorithms.
class PrintString
{
public:
PrintString(ostream &o = cout, char c = ' '):
os(o), sep(c) {
}
void operator()(const string &s) const
{
os << s << sep;
}
private:
ostream &os; // stream on which to write
char sep; // character to print after each output
};
string strText = "test";
PrintString printer; // uses the defaults; prints to cout
printer(strText); // prints s followed by a space on cout
PrintString printer2(cerr, '_');
printer2(strText); //cerr中打印s，后面跟一个换行符
//函数对象常常作为泛型算法的实参
std::vector<string> vecStr = {
"a1", "b1"};
for_each(vecStr.begin(), vecStr.end(), PrintString(cerr, '-'));
Output result:
test test_a1-b1-
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