[analysis of STL source code] imitation function (to be supplemented)

7.1 functor （fuctors） An overview

Function of imitative function ：

STL Various algorithms provided in , There are often two versions , One is the most intuitive default operation , The second is to run users with “template Parameter to specify the policy to be adopted ” . for example accumulate(), The first version is to add all the elements in the specified range , The second version can pass in the user specified operation , To replace this addition process .

This specified operation should be passed into the algorithm as a parameter of the algorithm , The only way is to design the operation as a function , then A function pointer As a parameter of the algorithm ; Or design the operation as a functor （ It's essentially a class object ）, Then create an object with this functor and pass it into the function as the parameter of the function .

The essence of a functor is an object that behaves like a function , In order to simulate the function call , Functors must be overloaded function call Operator , That is, parentheses 、operator(), So we can add (), Thus, we can call the operator().

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STL Classification of affine functions ：

• According to the number of operands ： One yuan 、 binary
• According to the function ： Arithmetic operations 、 Relationship between operation 、 Logical operations

Any program that wants to use STL Built in affine function , Must contain < functional > The header file .

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How to use the functor ：

7.2 Can be matched with （Adaptable） The key to

STL A functor should have a function adapter （function adapter） Ability to modify , Therefore, each functor must define its own corresponding type , Just like the five corresponding types defined by the iterator , These corresponding types are to enable the adapter to get some information of the functor . The corresponding types are typedef operation , This operation is completed at compile time , It will not affect the execution efficiency of the program , Without any additional burden .

The corresponding types of functors are mainly used to represent Function parameter type and Return value type ,< stl_function.h > The base classes of univariate and bivariate affine functions are defined respectively unary_function、binary_function, Any affine function should inherit one of them according to personal needs , They automatically have those corresponding types , It will automatically have the matching ability .

7.2.1 unary_function

unary_function Used to present the parameter type and return value type of unary functor ：

7.2.2 binary_function

binary_function The type of the first parameter used to render a bivariate functor 、 The type of the second parameter and the return value type ：