C++ Tips : String

C++ Tips

std::string, std::string_view

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std::string

• capacity increases but does not decrease.
• if you want to reduce capacity, use shrink_to_fit()

std::string s;

for(100'000)
	s.push_back('A');
// there is more than one reallocation.
// if you know the size, use reserve() first

#include <iostream>
#include <string>

int main() 
{
	std::string str;
	char arr[] = "Hello World!";

	str.assign(arr, 5);
	std::cout << str << '\n'; // Hello

	std::string s1(12, 'A');
	std::cout << s1 << '\n'; // AAAAAAAAAAAA

	std::string s2{52, 'A'};
	std::cout << s2 << '\n'; // 4A
}

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std::string::npos

• used to indicate that no position found.
• functions like .find(), .rfind(), .find_first_of() return an index if found.
• otherwise they return npos.
• prefer to use .at() rather than operator[]
• operator[] won’t thow. if pos > size, the behaviour is undefined.
• .at() will throw std::out_of_range, if pos >= size.

---

careful with .c_str()

#include <iostream>
#include <string>

int main() 
{
	std::string str = "Hi ";
	auto p = str.c_str();
	str += "Hello World! Hello World! Hello World! Hello World!";

	std::cout << p << '\n'; 
	// the behaviour is undefined, if reallocation happens.
}

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SSO

• Short String Optimization
• Short strings are stored directly inside the std::string without needing mem alloc.
• It saves both mem allocs and cpu cycles.
• Buffer’s size depends on compiler and platform.
• SSO is just SBO for strings

// std::string looks something like this
struct StringImpl {
    union {
        char* heap_ptr;          // for large strings
        char sso_buffer[15];     // for short strings (implementation-defined size)
    };
    size_t size;
    size_t capacity;
};

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SBO

• Small Buffer Optimization
• A class stores small data directly inside itself, avoiding heap allocation for small sizes (usually a few bytes).
• It saves both mem allocs and cpu cycles.
• Buffer’s size depends on compiler and platform.

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How to detect SBO in action?

• Use sizeof(std::function<…>) to check internal buffer size.
• Use tools like Valgrind
• Overload new and delete to log

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std::string_view

• #include <string_view>
• C++17
• Read-only.
• A non-owning view of a string.
• No allocate/copy memory.
• Just pointing the data.
• Faster, really faster.
• Dangling ref may happen, so be careful!

std::string_view sv;
{
    std::string temp = "hello";
    sv = temp;  
} 

// temp is destroyed so undefined behavior happens.
std::cout << sv << "\n";

Prefer to use swap functions for strings and containers. It is very efficient as it only changes pointers. Also use STL algorithms as much as possible.

std::string str;
// by this, you can read an entire line of text (including spaces)
getline(std::cin, str);

// Example: str -> Hello World