The short story: in C++ code using std::string
never use string literals except to initialize static-scoped std::string
constants.
(And by the way: NeverUseStaticLocalVariables
The long story: std::string
is all about avoiding copies. Reference counting and copy-on-write serve to maximise the sharing of a single heap-allocated char array while maintaining memory safety. When used consistently in a program it works rather nicely.
However, when mixed with classic C-style string literals std::string
can actually cause needless heap-allocated copies. Consider these innocent looking constructs:
void f(const std::string& s); void g(const std::string& s = "hello"); std::string h() { return "foo"; } void copy_surprise { std::string x = "x"; // 1 f("y"); // 2 g(); // 3 x = h(); //4 while (x != "end") { ... } // 4 }
Lines 1-4 all cause creation and destruction of an implicit temporary std::string
to hold the literal value. Line 5 does this for every execution of the while loop. That's a new/memcpy/delete each time. The heap is a heavily used resource, in tight inner loops in multi-threaded code this can be a severe contention bottleneck that cripples scalability.
Use static class std::string
constants or file-private constants instead. You can make global declarations file-private by using a nameless namespace (this is preferred over the use of the static
keyword.)
namespace { const std::string end("end"); } void f() { std::string x; while (x != end) {...} }
And once again NeverUseStaticLocalVariables