Difference between `push_back` and `+` in string

In C++, std::string is a dynamic container similar to std::vector, meaning that its operations have similar performance characteristics. Let’s define the following variables:

• s is a std::string
• c is a char

When appending characters to a string using push_back or the + operator, their efficiency differs based on how the operation is performed.

Characteristics of push_back

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The push_back(c) function appends a character to the end of the string. This operation is typically $O(1)$ because it only modifies the existing storage. However, if the string’s capacity is exceeded, a new memory allocation is triggered, causing an $O(n)$ complexity due to copying the existing content to the new space.

Characteristics of the + Operator

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Using + to concatenate strings or characters has different performance characteristics depending on where the new character is inserted:

• s = s + c (Appending a character to the end of the string): This is usually $O(1)$ if there is enough capacity in the string to accommodate the new character. However, if reallocation is needed, the complexity increases to $O(n)$ due to memory allocation and copying.
• s = c + s (Prepending a character to the beginning of the string): This operation requires creating a new std::string instance, copying c, and then appending s to it. Since the entire original string must be copied into new storage, the complexity is $O(n)$.

Summary

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Key Takeaways:

• Appending a character to the end of a string (s + c or push_back) is typically $O(1)$.
• Prepending a character (c + s) creates a new string, making it $O(n)$.
• If capacity is exceeded, any modification may trigger reallocation, leading to $O(n)$ complexity.

Understanding these performance differences is crucial when optimizing string operations, especially in scenarios involving frequent concatenation. To improve efficiency, avoid inserting characters at the beginning of a string whenever possible to minimize unnecessary memory allocations and copies.

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