Question

From the documentation of the StringPiece class in Chromium’s source code:

// A string-like object that points to a sized piece of memory.
//
// Functions or methods may use const StringPiece& parameters to accept either
// a "const char*" or a "string" value that will be implicitly converted to
// a StringPiece.  
//
// Systematic usage of StringPiece is encouraged as it will reduce unnecessary
// conversions from "const char*" to "string" and back again.

Example of use:

void foo(StringPiece const & str) // Pass by ref. is probably not needed
{
   // str has same interface of const std::string
}

int main()
{
    string bar("bar");
    foo(bar); // OK, no mem. alloc.

    // No mem. alloc. either, would be if arg. of "foo" was std::string
    foo("baz");  
}

This seems like such an important and obvious optimization that I can’t understand why it’s not more widespread, and why a class similar to StringPiece is not already in the standard.

Are there any reasons why I shouldn’t replace the use of string and char* parameters in my own code with this class? Is there anything like it already in the C++ standard libraries?

UPDATE. I’ve learnt that LLVM’s source uses a similar concept: the StringRef class.

Answer 1

Because why bother? With copy elision and/or pass by reference, memory allocations for std::string can usually be avoided as well.

The string situation in C++ is confusing enough as it is, without adding still more string classes.

If the language was to be redesigned from scratch, or if backwards compatibility wasn’t an issue, then this is one of many possible improvements that could be done to string handling in C++. But now that we’re stuck with both char* and std::string, adding a stringref-style class into the mix would cause a lot of confusion, with limited benefit.

Apart from this, isn’t the same effect achieved more idiomatically with a pair of iterators?
If I want to pass a sequence of characters, whether they belong to a string or a char*, why shouldn’t I just use a pair of iterators to delimit them?