How to Resize a std::string in C++ Without Changing Its Capacity

When working with std::string in C++, resizing it is straightforward, but ensuring its capacity (the allocated memory size) stays the same can be confusing. Let’s break down how to resize a string while preserving its capacity—no jargon, just clarity.

What Are size() and capacity()?

• size(): The number of characters currently in the string.

• capacity(): The total memory allocated to the string (always ≥ size()).

For example:

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std::string str = "Hello";  
std::cout << str.size();      // Output: 5  
std::cout << str.capacity();  // Output: 15 (varies by compiler)  

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How Resizing Affects Capacity

The resize() method changes the size() of the string. However:

1. If you increase the size beyond the current capacity(), the string reallocates memory, increasing capacity.

2. If you decrease the size, the capacity() usually stays the same (no memory is freed by default).

Example:

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std::string str = "Hello";  
str.reserve(20);  // Force capacity to 20  
str.resize(10);   // Size becomes 10, capacity remains 20  
str.resize(25);   // Size 25, capacity increases (now ≥25)  

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How to Resize a String and Keep the Same Capacity

To avoid changing the capacity, ensure the new size() does not exceed the current capacity():

Step 1: Check the current capacity.

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size_t current_cap = str.capacity();  

Step 2: Resize within the current capacity.

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str.resize(new_size);  // Only works if new_size ≤ current_cap  

Full Example:

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#include <iostream>  
#include <string>  

int main() {  
    std::string str = "C++ is fun!";  
    str.reserve(50);  // Set capacity to 50  

    std::cout << "Original capacity: " << str.capacity() << "\n"; // 50  

    // Resize to 20 (within capacity)  
    str.resize(20);  
    std::cout << "New size: " << str.size() << "\n";      // 20  
    std::cout << "Capacity remains: " << str.capacity();  // 50  

    return 0;  
}  

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What If You Want to Shrink Capacity?

If you want to reduce capacity to match the new size (free memory), use shrink_to_fit():

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str.resize(10);  
str.shrink_to_fit();  // Capacity ≈10 (exact value depends on the compiler)  

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Key Takeaways

1. Resize Safely: To keep the same capacity, ensure new_size ≤ current_capacity.

2. Avoid Reallocation: Use reserve() upfront to pre-allocate memory if you know the maximum size needed.

3. Shrink Explicitly: Use shrink_to_fit() only if you need to free unused memory (rarely necessary).

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Why Does Capacity Matter?

• Performance: Frequent reallocations (capacity changes) slow down your code.

• Memory Efficiency: Over-reserving wastes memory; under-reserving causes reallocations.

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Final Answer

Yes! To resize a std::string without changing its capacity:

1. Check the current capacity().

2. Resize to a value ≤ that capacity.

Use reserve() to control capacity upfront and avoid surprises. Happy coding! 🚀