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018-string-view-avoiding-copies.md

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std::string_view to Eliminate String Copies

Problem

C++ code that passes strings through multiple function layers (parsing, validation, transformation) where each function takes const std::string& or std::string. Even with const&, callers with string literals or substrings must construct a temporary std::string, triggering heap allocation. The metric was throughput of a text parsing pipeline processing millions of short strings.

What Worked

Replacing const std::string& parameters with std::string_view in functions that only read (never store or modify) the string. string_view is a non-owning (pointer, length) pair — 16 bytes, always on the stack, zero allocation. It binds to std::string, const char*, string literals, and substrings without any copy or allocation.

For a CSV parser calling 6 functions per field (validate, trim, classify, transform, hash, store), switching the first 5 functions from const string& to string_view eliminated 5 temporary string allocations per field. On a 500MB CSV with 50M fields, parsing throughput improved 1.9x.

Experiment Data

Parameter Type Parse Throughput (MB/s) Heap Allocs/field
const string& 180 6
string_view (read path) 340 1 (final store only)

Code Example

// BEFORE: every call site may allocate
bool validate(const std::string& field);
std::string trim(const std::string& field);

// AFTER: zero-allocation read path
bool validate(std::string_view field);
std::string_view trim(std::string_view field); // return view into original buffer

// Substring without allocation:
std::string_view line = "name,age,city";
auto field = line.substr(5, 3); // "age" — no allocation, just pointer arithmetic

// DANGER: string_view must not outlive the data it points to
std::string_view dangling() {
    std::string temp = compute();
    return temp; // BUG: temp destroyed, view dangles
}

What Didn't Work

  • Replacing parameters that store the string: Functions that save the string into a member variable or container still need std::string (ownership). Using string_view there requires an explicit conversion (std::string(sv)), which allocates anyway — but now the allocation is hidden and easy to miss.
  • string_view for null-terminated C APIs: string_view is not null-terminated. Passing .data() to C functions expecting null termination is undefined behavior unless the view happens to point at a null-terminated source.

Environment

C++17 (where std::string_view was introduced). Applies to all implementations. GCC/Clang/MSVC all optimize string_view to two registers.