What is string_view?

Understanding std::string_view in C++17 and Beyond

Explore std::string_view, a non-owning reference to a string, and learn how it improves performance and safety in C++ applications by avoiding unnecessary copies.

Before C++17, handling string data often involved either passing const std::string& or const char* to functions. While const std::string& avoids copies, it still implies ownership or potential heap allocation. const char* is lightweight but lacks length information, making it prone to errors and requiring null termination. std::string_view, introduced in C++17 (and available earlier via the Fundamentals TS), addresses these issues by providing a lightweight, non-owning reference to a sequence of characters. It's essentially a pointer to the beginning of a character sequence and its length, without managing the underlying memory.

What is std::string_view?

std::string_view is a class template that provides a read-only view into an existing character sequence. It does not own the characters it refers to; it merely points to them. This means that constructing a string_view from an std::string or a C-style string (const char*) does not involve any memory allocation or copying of the character data. It's a 'view' into the data, much like a pointer and a size. This characteristic makes it incredibly efficient for functions that only need to read string data without modifying it.

classDiagram
    class std::string {
        - char* data
        - size_t length
        - size_t capacity
        + std::string(const char*)
        + ~std::string()
    }
    class std::string_view {
        - const char* ptr
        - size_t len
        + std::string_view(const char*, size_t)
        + std::string_view(const std::string&)
    }
    std::string <.. std::string_view : views

Relationship between std::string and std::string_view

Key Benefits and Use Cases

The primary advantage of std::string_view is performance. By avoiding copies, it reduces memory allocations and CPU cycles, especially when dealing with large strings or frequent string manipulations. It's ideal for function parameters where the function only needs to read the string content. For example, a logging function or a parser that extracts substrings can greatly benefit from using string_view.

#include <iostream>
#include <string>
#include <string_view>

void process_string(std::string_view sv) {
    std::cout << "Processing: '" << sv << "' (length: " << sv.length() << ")\n";
}

int main() {
    std::string s = "Hello, world!";
    const char* c_str = "C-style string";

    process_string(s); // No copy from std::string
    process_string(c_str); // No copy from const char*
    process_string("Literal string"); // No copy from string literal

    // Substring operations are also views
    std::string_view sub_sv = s.substr(0, 5);
    process_string(sub_sv); // Still no copy

    return 0;
}

Demonstrating std::string_view with various string types

Important Considerations and Pitfalls

While std::string_view offers significant benefits, its non-owning nature introduces a critical caveat: dangling views. Since string_view does not manage the lifetime of the underlying character data, it's crucial to ensure that the data outlives the string_view itself. If the underlying string is destroyed or goes out of scope while the string_view still exists, accessing the string_view will result in undefined behavior. This is similar to the dangers of dangling pointers.

flowchart TD
    A[Create std::string] --> B{Create std::string_view from A}
    B --> C[Use std::string_view]
    C --> D{std::string A goes out of scope?}
    D -- Yes --> E[std::string_view becomes dangling!]
    D -- No --> F[std::string_view remains valid]

Lifecycle of std::string_view and potential dangling issues

#include <iostream>
#include <string>
#include <string_view>

std::string_view get_temporary_view() {
    std::string temp_str = "Temporary data";
    // DANGER: temp_str will be destroyed when this function returns
    return temp_str; // Returns a dangling string_view
}

int main() {
    std::string_view dangling_sv = get_temporary_view();
    // Accessing dangling_sv here is UNDEFINED BEHAVIOR!
    // std::cout << dangling_sv << std::endl; // CRASH or garbage output

    std::string_view valid_sv;
    {
        std::string s = "Valid data";
        valid_sv = s; // valid_sv is valid within this scope
        std::cout << "Inside scope: " << valid_sv << std::endl;
    } // 's' is destroyed here
    // std::cout << "Outside scope: " << valid_sv << std::endl; // DANGER: dangling_sv here too!

    return 0;
}

Example of dangling std::string_view leading to undefined behavior