std::string concatenation in C++

Question

What is the principle of std::string concatenation in C++ ? How does it works in memory allocation ?

I found out while exploring a leetcode card that in Java:
"concatenation works by first allocating enough space for the new string, copy the contents from the old string and append to the new string".
And unlike Java, "there is no noticeable performance impact in C++".

I thought that it is the same in C++ because of how dynamic arrays work. As I remember if you want to increase the capacity of a dynamic array, the program first creates a new space, the size of which is equal to the desired size, and then copies all the elements of old array to the new.

3CEZVQ · Accepted Answer · 2024-11-13 15:38:07Z

There are two operators in C++ that can be used for std::string concatenation:

Binary operator std::string result = a + b compiles to:

result.reserve(a.size() + b.size());
result.append(a);
result.append(b);

Assignment operator a += b compiles to:

a.append(b);

Both of these may result in memory allocation, unless the result string happens to have enough allocated space already. This may be either due to prior allocation such as by reserve() call, or due to small string optimization that stores strings up to about 15 bytes directly in the structure on the stack.

The significant difference between Java and C++ is the += operator (or .append()) call, which modifies the string in place. Java strings are immutable so this is not available there. When modifying the existing string, C++ usually allocates more space than immediately needed, so that every call does not result in a new memory allocation. Then the existing part of the string does not need to be copied, only the appended part.

The claim that "there is no noticeable performance impact in C++" is not true if you have a lot of strings or they are long. Unlike Java where StringBuilder gives a speed advantage, C++ does not have a direct equivalent. Calling .reserve() ahead of time will give roughly equivalent performance. For best performance you would reorganize the code to completely avoid the copying required for appending, such as by using a rope data structure.

wohlstad · Accepted Answer · 2024-11-13 12:19:52Z

In principle, std::string concatenation in C++ works similar to what you read about Java:
You need to have enough space available and then copy into it the content of the 2 strings you are concatenating.

However - although it is not mandated by the standard most (if not all) standard library implementations for std::string include a mechanism called small string optimization. It allows to avoid dynamic allocation for the string content if it is small enough, by embdding an small array of characters directly in the std::string object.
Note that "enough" is implementation dependant.

This means that if you concatenate 2 small enough strings sch as the result is still small enough - no dynamic allocation will be done.

But you will still need to copy the content of both strings into the result.

Collectives™ on Stack Overflow

std::string concatenation in C++

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related