A function is a group of statements that together perform a task. Every C++ program has at least one function, which is main(), and all the most trivial programs can define additional functions.
You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is such that each function performs a specific task.
A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.
The C++ standard library provides numerous built-in functions that your program can call. For example, function strcat() to concatenate two strings, function memcpy() to copy one memory location to another location and many more functions.
A function is known with various names like a method or a sub-routine or a procedure etc.
Defining a Function
The general form of a C++ function definition is as follows โ
return_type function_name( parameter list ) {
body of the function
}
A C++ function definition consists of a function header and a function body. Here are all the parts of a function โ
- Return Type โ A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
- Function Name โ This is the actual name of the function. The function name and the parameter list together constitute the function signature.
- Parameters โ A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
- Function Body โ The function body contains a collection of statements that define what the function does.
Example
Following is the source code for a function called max(). This function takes two parameters num1 and num2 and return the biggest of both โ
// function returning the max between two numbers
int max(int num1, int num2) {
// local variable declaration
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Function Declarations
A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.
A function declaration has the following parts โ
return_type function_name( parameter list );
For the above defined function max(), following is the function declaration โ
int max(int num1, int num2);
Parameter names are not important in function declaration only their type is required, so following is also valid declaration โ
int max(int, int);
Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.
Calling a Function
While creating a C++ function, you give a definition of what the function has to do. To use a function, you will have to call or invoke that function.
When a program calls a function, program control is transferred to the called function. A called function performs defined task and when itโs return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.
To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value. For example โLive Demo
#include <iostream>
using namespace std;
// function declaration
int max(int num1, int num2);
int main () {
// local variable declaration:
int a = 100;
int b = 200;
int ret;
// calling a function to get max value.
ret = max(a, b);
cout << "Max value is : " << ret << endl;
return 0;
}
// function returning the max between two numbers
int max(int num1, int num2) {
// local variable declaration
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
I kept max() function along with main() function and compiled the source code. While running final executable, it would produce the following result โ
Max value is : 200
Function Arguments
If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.
The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.
While calling a function, there are two ways that arguments can be passed to a function โ
| Sr.No | Call Type & Description |
|---|---|
| 1 | Call by ValueThis method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. |
| 2 | Call by PointerThis method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument. |
| 3 | Call by ReferenceThis method copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument. |
By default, C++ uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.
Default Values for Parameters
When you define a function, you can specify a default value for each of the last parameters. This value will be used if the corresponding argument is left blank when calling to the function.
This is done by using the assignment operator and assigning values for the arguments in the function definition. If a value for that parameter is not passed when the function is called, the default given value is used, but if a value is specified, this default value is ignored and the passed value is used instead. Consider the following example โLive Demo
#include <iostream>
using namespace std;
int sum(int a, int b = 20) {
int result;
result = a + b;
return (result);
}
int main () {
// local variable declaration:
int a = 100;
int b = 200;
int result;
// calling a function to add the values.
result = sum(a, b);
cout << "Total value is :" << result << endl;
// calling a function again as follows.
result = sum(a);
cout << "Total value is :" << result << endl;
return 0;
}
When the above code is compiled and executed, it produces the following result โ
Total value is :300
Total value is :120
Hร m trong C++
Mแปt hร m lร mแปt nhรณm cรกc cรขu lแปnh cรนng nhau thแปฑc hiแปn mแปt nhiแปm vแปฅ. Mแปi chฦฐฦกng trรฌnh C++ cรณ รญt nhแบฅt mแปt hร m, lร hร mย main().
Bแบกn cรณ thแป chia mรฃ cแปงa bแบกn thร nh cรกc hร m riรชng biแปt. Cรกch bแบกn phรขn chia mรฃ cแปงa bแบกn giแปฏa cรกc hร m khรกc nhau tรนy thuแปc vร o bแบกn, nhฦฐng vแป mแบทt logic mแปi hร m thแปฑc hiแปn mแปt tรกc vแปฅ cแปฅ thแป.
Mแปt khai bรกo hร m cho trรฌnh biรชn dแปch biแบฟt vแป tรชn, kiแปu trแบฃ vแป vร cรกc tham sแป cแปงa hร m. ฤแปnh nghฤฉa hร m cung cแบฅp phแบงn thรขn thแปฑc cแปงa hร m.
Thฦฐ viแปn chuแบฉn C++ cung cแบฅp nhiแปu hร m ฤฦฐแปฃc tรญch hแปฃp sแบตn mร chฦฐฦกng trรฌnh cแปงa bแบกn cรณ thแป gแปi. Vรญ dแปฅ, strcat() ฤแป nแปi hai chuแปi, memcpy() ฤแป sao chรฉp mแปt vแป trรญ bแป nhแป sang mแปt vแป trรญ khรกc vร nhiแปu hร m khรกc.
Hร m trong C++ cรฒn ฤฦฐแปฃc gแปi lร thแปง tแปฅc hoแบทc chฦฐฦกng trรฌnh con trong cรกc ngรดn ngแปฏ lแบญp trรฌnh khรกc.
ฤแป thแปฑc hiแปn bแบฅt kแปณ tรกc vแปฅ nร o, chรบng ta cรณ thแป tแบกo ra cรกc hร m. Mแปt hร m cรณ thแป ฤฦฐแปฃc gแปi nhiแปu lแบงn. Nรณ cung cแบฅp tรญnh mรด ฤun vร khแบฃ nฤng sแปญ dแปฅng lแบกi mรฃ. Hร m giรบp phรขn chia vแบฅn ฤแป phแปฉc tแบกp thร nh cรกc thร nh phแบงn nhแป giรบp chฦฐฦกng trรฌnh dแป hiแปu vร dแป sแปญ dแปฅng.
Lแปฃi thแบฟ cแปงa cรกc hร m trong C++
1. Tรกi sแปญ dแปฅng mรฃ
Bแบฑng cรกch tแบกo cรกc hร m trong C++, bแบกn cรณ thแป gแปi nรณ nhiแปu lแบงn. Vรฌ vแบญy, bแบกn khรดng cแบงn phแบฃi viแบฟt cรนng mแปt mรฃ mแปt hoแบทc nhiแปu lแบงn nแปฏa.
2. Tแปi ฦฐu hรณa mรฃ
Nรณ lร m cho mรฃ ฤฦฐแปฃc tแปi ฦฐu hรณa, chรบng ta khรดng cแบงn phแบฃi viแบฟt nhiแปu mรฃ.
Giแบฃ sแปญ, bแบกn phแบฃi kiแปm tra 3 sแป (531, 883 vร 781) cรณ phแบฃi lร sแป nguyรชn tแป hay khรดng. Khรดng sแปญ dแปฅng hร m, bแบกn cแบงn viแบฟt logic sแป nguyรชn tแป 3 lแบงn. Vรฌ vแบญy, cรณ sแปฑ lแบทp lแบกi cแปงa mรฃ.
Nhฦฐng nแบฟu bแบกn sแปญ dแปฅng cรกc hร m, bแบกn chแป cแบงn viแบฟt logic mแปt lแบงn vร bแบกn cรณ thแป sแปญ dแปฅng lแบกi nรณ nhiแปu lแบงn.
ฤแปnh nghฤฉa mแปt hร m
Dแบกng chung cแปงa ฤแปnh nghฤฉa hร m trong C++ nhฦฐ sau:
return_type function_name(parameter list) {
// code
}
ฤแปnh nghฤฉa hร m trong lแบญp trรฌnh C++ bao gแปm tรชn hร m vร phแบงn thรขn hร m . Dฦฐแปi ฤรขy lร tแบฅt cแบฃ cรกc phแบงn cแปงa hร m:
- Kiแปu trแบฃ vแป: Mแปt hร m cรณ thแป trแบฃ vแป mแปt giรก trแป. Cรกc return_type lร kiแปu dแปฏ liแปu cแปงa giรก trแป hร m trแบฃ vแป. Mแปt sแป hร m thแปฑc hiแปn cรกc hoแบกt ฤแปng mong muแปn mร khรดng trแบฃ vแป mแปt giรก trแป. Trong trฦฐแปng hแปฃp nร y, return_type lร tแปซ khรณa void.
- Tham sแป: Mแปt tham sแป giแปng nhฦฐ mแปt trรฌnh giแปฏ chแป. Khi mแปt hร m ฤฦฐแปฃc gแปi, bแบกn chuyแปn mแปt giรก trแป cho tham sแป. Giรก trแป nร y ฤฦฐแปฃc gแปi lร tham sแป hoแบทc ฤแปi sแป thแปฑc tแบฟ. Danh sรกch tham sแป tham chiแบฟu ฤแบฟn loแบกi, thแปฉ tแปฑ vร sแป tham sแป cแปงa hร m. Cรกc tham sแป lร tรนy chแปn; cรณ nghฤฉa lร , mแปt hร m cรณ thแป khรดng chแปฉa tham sแป.
- Thรขn hร m: Phแบงn thรขn hร m chแปฉa mแปt tแบญp hแปฃp cรกc cรขu lแปnh xรกc ฤแปnh chแปฉc nฤng cแปงa hร m.
Vรญ dแปฅ hร m trong C++
Vรญ dแปฅ dฦฐแปi ฤรขy lร mรฃ nguแปn cho mแปt hร m ฤฦฐแปฃc gแปi lร max(). Hร m nร y truyแปn vร o hai tham sแป num1 vร num2 vร trแบฃ vแป giรก trแป lแปn nhแบฅt giแปฏa hai tham sแป:
/* hร m trแบฃ vแป giรก trแป lแปn nhแบฅt giแปฏa 2 sแป */
int max(int num1, int num2) {
/* khai bรกo biแบฟn local */
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Khai bรกo hร m trong C++
Mแปt khai bรกo hร m cho trรฌnh biรชn dแปch biแบฟt vแป tรชn hร m vร cรกch gแปi hร m. Cฦก thแป thแปฑc tแบฟ cแปงa hร m cรณ thแป ฤฦฐแปฃc ฤแปnh nghฤฉa riรชng.
Mแปt khai bรกo hร m cรณ cรกc phแบงn sau:
return_type function_name(parameter list);
ฤแปi vแปi hร m ฤฦฐแปฃc ฤแปnh nghฤฉa แป trรชn max (), khai bรกo hร m nhฦฐ sau:
int max(int num1, int num2);
Tรชn tham sแป khรดng quan trแปng trong khai bรกo hร m chแป loแบกi cแปงa chรบng lร bแบฏt buแปc, vรฌ vแบญy sau ฤรขy cลฉng lร khai bรกo hแปฃp lแป:
int max(int, int);
Khai bรกo hร m lร bแบฏt buแปc khi bแบกn ฤแปnh nghฤฉa mแปt hร m trong mแปt tแปp nguแปn vร bแบกn gแปi hร m ฤรณ trong mแปt tแปp khรกc. Trong trฦฐแปng hแปฃp nร y, bแบกn nรชn khai bรกo hร m แป ฤแบงu tแปp gแปi hร m.
Gแปi mแปt hร m trong C++
Hร m trong lแบญp trรฌnh C++ hoแบกt ฤแปng nhฦฐ thแบฟ nร o? Hรฌnh แบฃnh sau ฤรขy mรด tแบฃ gแปi mแปt hร m do ngฦฐแปi dรนng ฤแปnh nghฤฉa bรชn trong hร m main():
Trong khi tแบกo mแปt hร m C, bแบกn ฤฦฐa ra mแปt ฤแปnh nghฤฉa vแป chแปฉc nฤng cแปงa hร m. ฤแป sแปญ dแปฅng mแปt hร m, bแบกn sแบฝ phแบฃi gแปi hร m ฤรณ ฤแป thแปฑc hiแปn tรกc vแปฅ ฤฦฐแปฃc xรกc ฤแปnh.
Khi mแปt chฦฐฦกng trรฌnh gแปi mแปt hร m, ฤiแปu khiแปn chฦฐฦกng trรฌnh ฤฦฐแปฃc chuyแปn ฤแบฟn hร m ฤฦฐแปฃc gแปi. Mแปt hร m ฤฦฐแปฃc gแปi thแปฑc hiแปn mแปt nhiแปm vแปฅ ฤรฃ ฤแปnh nghฤฉa vร khi cรขu lแปnh trแบฃ vแป cแปงa nรณ ฤฦฐแปฃc thแปฑc hiแปn hoแบทc khi nรณ kแบฟt thรบc bแบฑng hร m ฤรณng, nรณ sแบฝ trแบฃ vแป chฦฐฦกng trรฌnh ฤiแปu khiแปn quay trแป lแบกi chฦฐฦกng trรฌnh chรญnh.
ฤแป gแปi mแปt hร m, bแบกn chแป cแบงn chuyแปn cรกc tham sแป bแบฏt buแปc cรนng vแปi tรชn hร m vร nแบฟu hร m trแบฃ vแป mแปt giรก trแป, thรฌ bแบกn cรณ thแป lฦฐu trแปฏ giรก trแป trแบฃ vแป. Vรญ dแปฅ:
#include <iostream>
using namespace std;
/* khai bao ham */
int max(int num1, int num2);
int main () {
/* dinh nghia bien local */
int a = 100;
int b = 200;
int ret;
/* goi mot ham de lay gia tri lon nhat */
ret = max(a, b);
cout << "Max value is: " << ret << endl;
return 0;
}
/* ham tra ve gia tri lon nhat giua hai so */
int max(int num1, int num2) {
/* dinh nghia bien local */
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
Kแบฟt quแบฃ:
Max value is : 200
ACE IT SKILLS 