Reserve vs Resize

The reserve() and resize() methods in std::string serve different purposes related to memory management and string size.

The reserve() method requests a change in the capacity of the string, which is the amount of allocated storage. It does not change the size (the number of characters in the string). For example:

1#include <iostream>
2#include <string>
3
4int main() {
5  std::string Greeting{"Hello"};
6  Greeting.reserve(20);  
7
8  std::cout << "Capacity: "
9    << Greeting.capacity() << '\n';
10  std::cout << "Size: "
11    << Greeting.size() << '\n';
12}

1Capacity: 31
2Size: 5

The resize() method changes the size of the string. If the new size is larger than the current size, it appends null characters (\0) or a specified character. If it's smaller, it truncates the string. For example:

1#include <iostream>
2#include <string>
3
4int main(){
5  std::string Greeting{"Hello"};
6  Greeting.resize(10, '!');
7
8  std::cout << Greeting;
9}

1Hello!!!!!

Purpose:

• reserve(): Adjusts the capacity, not the size.
• resize(): Adjusts the size and potentially the capacity.

Effect on String Content:

• reserve(): No effect on string content.
• resize(): May add or remove characters.

Usage Scenarios:

• reserve(): Optimize performance by pre-allocating memory when you know the string will grow.
• resize(): Directly change the number of characters in the string.

• reserve(): Helps avoid multiple allocations when the string grows, improving performance in scenarios where the final size is known in advance.
• resize(): Useful for quickly changing the string's size but can be less efficient if done frequently without need.

By understanding the differences between these methods, you can choose the right one based on your specific requirements and optimize your string manipulations in C++.