Why pointer to function

If there are no parameters, the parentheses can be left empty. Much like the auto keyword can be used to infer the type of normal variables, the auto keyword can also infer the type of a function pointer.

Function pointers are useful primarily when you want to store functions in an array or other structure , or when you need to pass a function to another function. Because the native syntax to declare function pointers is ugly and error prone, we recommend using std::function.

In places where a function pointer type is only used once e. In places where a function pointer type is used multiple times, a type alias to a std::function is a better choice to prevent repeating yourself.

Ensure the user enters a valid operation. Show Solution. These should take two integer parameters and return an integer.

Use std::function. Call the return value from that function with your inputs and print the result. Previous Post 8. Next Post Your email address will not be displayed. They have scope only inside the function. Hence if you return a pointer connected to a local variable, that pointer will be pointing to nothing when the function ends. It is possible to declare a pointer pointing to a function which can then be used as an argument in another function.

A pointer to a function is declared as follows,. A function pointer can point to a specific function when it is assigned the name of that function. Here s is a pointer to a function sum. Now sum can be called using function pointer s along with providing the required argument values. You will find a lot of complex function pointer examples around, lets see one such example and try to understand it.

Think of a functionoid object as a freeze-dried function-call emphasis on the word call. Unlike a pointer to a function, a functionoid is conceptually a pointer to a partially called function. Imagine for the moment a technology that lets you pass some-but-not-all arguments to a function, then lets you freeze-dry that partially completed call.

Pretend that technology gives you back some sort of magic pointer to that freeze-dried partially-completed function-call. Plus they allow not require you to change the freeze-dried state when it gets called, meaning functionoids can remember information from one call to the next.

Suppose the original functions in the old-fashioned function-pointer style took slightly different parameters. With functionoids, the situation is, at least sometimes, much better. Another benefit of functionoids is apparent if we change the example from an array of functionoids to a local functionoid. The sort or binarySearch routine is called childRoutine and the comparison function-pointer type is called FunctPtr :.

Then different callers would pass different function-pointers depending on what they thought was best:. Given this example as a backdrop, we can see two benefits of functionoids over function-pointers. With plain function-pointers, people normally maintain state between calls via static data. However static data is not intrinsically thread-safe — static data is shared between all threads.

The functionoid approach provides you with something that is intrinsically thread-safe since the code ends up with thread-local data. The functionoid approach gives you a third option which is not available with the old-fashioned approach: the functionoid lets callers decide whether they want thread-local or thread-global data. In the previous FAQ , functionoids were implemented using virtual functions and will typically cost you a function-call.

An alternate approach uses templates. The following example is similar in spirit to the one in the previous FAQ. Think of it as passing in a parameter: if you know at compile-time the kind of functionoid you ultimately want to pass in, then you can use the above technique, and you can, at least in typical cases , get a speed benefit from having the compiler inline-expand the functionoid code within the caller. Here is an example:. When the compiler compiles the above, it might inline-expand the call which might improve performance.

Aside: as was hinted at in the first paragraph above, you may also pass in the names of normal functions though you might incur the cost of the function call when the caller uses these :.

This article will explain how the function pointer works and give you several examples, as well as code samples. Because the function pointer can accept many parameters, it can point to any function that accepts two integer arguments and returns void.

There is no need for us to call the function. We only listed the name of the function, no brackets, and no parameters to print the address of the main function. With the use of a function pointer, we may use the name of a function to call it.

The syntax for calling the function via the function pointer is the same as for calling it directly. Go ahead and run the code here Output:. This means that the address of the add method is stored in funcptr.

We can now use funcptr to invoke the add method. The add function is called by the phrase funcptr 7,10 , and the result is put in the sum variable.