Difference between concatenation of strings using (str += s) and (str = str + s)

A string is a collection of characters. For example, "GeeksforGeeks" is a string. C++ provides primitive data types to create a string. The string can also be initialized at the time of declaration.

Syntax:

string str;

string str = "GeeksforGeeks"

Here, "GeeksforGeeks" is a string literal.

This article shows the difference between the concatenation of the strings using the addition assignment operator (+=) and the addition (+) operator used with strings. Concatenation is the process of joining end-to-end.

In C++, a string addition assignment operator is used to concatenate one string to the end of another string.

Syntax:

str += value

Here, 
value is a string to be concatenated with str.

It appends the value (literal) at the end of the string, without any reassignment.

Example: Below is the C++ program to demonstrate the addition assignment operator.

GeeksforGeeks

In C++, a string addition operator is used to concatenate one string to the end of another string. But in this case the after the concatenation of strings, the modified string gets assigned to the string.

Syntax:

str = str + value

Here, 
value is a string to be concatenated with str.

It firstly appends the value (literal) at the end of the string and then reassigns it to str.

Example: Below is the C+ program to demonstrate the above approach.

GeeksforGeeks

Although both operators when used with strings can be used for the concatenation of strings, there are some differences between them:

Factor 1: Assignment of the modified string:

• The addition assignment operator (+=) concatenates two strings by appending one string at the end of another string.
• The addition operator(+) concatenates two strings by appending one string at the end of the original string and then assigning the modified string to the original string.

Example: Below is the C++ program to demonstrate the above approach.

Resultant string using += GeeksforGeeks
Resultant string using + GeeksforGeeks

Factor 2: Operator overloaded functions used:

• The addition assignment operator (+=) concatenates two strings because the operator is overloaded internally.
• In this case, also, the addition operator (+) concatenates two strings because the operator is overloaded internally.

Factor 3: Number of strings concatenated:

• The addition assignment operator (+=) can concatenate two strings at a time in a single statement.
• The addition operator (+) can concatenate multiple strings by using multiple addition (+) operators between the string in a single statement. For example, str = str₁ + str₂ + str₃ + ... + str_n

Example: In this program, three different statements are required to concatenate three strings; str, str1, str2, and str3 using the assignment addition operator (+=) and a single statement is required to concatenate three strings; str, str1, str2, and str3 using the addition operator (+). 

Resultant string using +=GeeksforGeeks GeeksforGeeks GeeksforGeeks GeeksforGeeks
Resultant string using + GeeksforGeeks GeeksforGeeks GeeksforGeeks GeeksforGeeks

Factor 4: Performance:

• The addition assignment operator (+=) when used for the concatenation of strings gives better efficiency as compared to the addition(+) operator. This is because no reassignment of strings takes place in this case.
• The addition operator (+)  when used for the concatenation of strings, is less efficient as compared to the addition (+=) operator. This is because the assignment of strings takes place in this case.

Example: Below is the program to demonstrate the performance of the += string concatenation method.

Time taken by program is : 0.000490 sec

Example: Below is the program to demonstrate the performance of the + string concatenation method.

Time taken by program is : 0.000715 sec