stl · medium

Small String Optimization (SSO)

std::string uses Small String Optimization: short strings are stored in an inline buffer inside the object, so common short strings cost zero allocations and stay cache-friendly. Once the string exceeds the inline capacity (~15 chars typically), it allocates a heap buffer and the object reinterprets its storage as {pointer, size, capacity}. sizeof(std::string) is fixed either way.

🔑 Key line

Small String Optimization: a std::string keeps short strings (~15 chars) inside the object itself — no heap. Longer strings allocate on the heap; sizeof(std::string) never changes.

The code

std::string s;                 // empty: cap ~15, no heap
s = "hi";                      // 2 chars: lives INLINE
s = "exactly_15_char";         // 15 chars: still inline (at limit)
s = "long enough to overflow"; // > 15 chars: HEAP allocation
// now: s.data() points to the heap; sizeof(s) is unchanged

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