Macro for static std::string object from literal - c++

Question

Suppose I need to call a function foo that takes a const std::string reference from a great number of places in my code:
int foo(const std::string&);
..
foo("bar");
..
foo("baz");
Calling a function with a string literal like this will create temporary std::string objects, copying the literal each time.
Unless I'm mistaken, compilers won't optimize this by creating a static std::string object per literal that can be reused for subsequent calls. I know that g++ has advanced string pool mechanisms, but I don't think it extends to the std::string objects themselves.
I can do this "optimization" myself, which makes the code somewhat less readable:
static std::string bar_string("bar");
foo(bar_string);
..
static std::string baz_string("baz");
foo(baz_string);
Using Callgrind, I can confirm that this does indeed speed up my program.
I thought I'd try to make a macro for this, but I don't know if it's possible. What I would want is something like:
foo(STATIC_STRING("bar"));
..
foo(STATIC_STRING("baz"));
I tried creating a template with the literal as a template parameter, but that proved impossible. And since a function definition in a code block isn't possible, I'm all out of ideas.
Is there an elegant way of doing this, or will I have to resort to the less readable solution?

Answer 1

If that function foo does not make a copy of the string then its interface is sub-optimal. It is better to change it to accept char const* or string_view, so that the caller is not required to construct std::string.
Or add overloads:
void foo(char const* str, size_t str_len); // Does real work.
inline void foo(std::string const& s) { foo(s.data(), s.size()); }
inline void foo(char const* s) { foo(s, strlen(s)); }

Answer 2

You may use something like that to create your static std::string "in place":
#include <cstdint>
#include <string>
// Sequence of char
template <char...Cs> struct char_sequence
{
template <char C> using push_back = char_sequence<Cs..., C>;
};
// Remove all chars from char_sequence from '\0'
template <typename, char...> struct strip_sequence;
template <char...Cs>
struct strip_sequence<char_sequence<>, Cs...>
{
using type = char_sequence<Cs...>;
};
template <char...Cs, char...Cs2>
struct strip_sequence<char_sequence<'\0', Cs...>, Cs2...>
{
using type = char_sequence<Cs2...>;
};
template <char...Cs, char C, char...Cs2>
struct strip_sequence<char_sequence<C, Cs...>, Cs2...>
{
using type = typename strip_sequence<char_sequence<Cs...>, Cs2..., C>::type;
};
// struct to create a std::string
template <typename chars> struct static_string;
template <char...Cs>
struct static_string<char_sequence<Cs...>>
{
static const std::string str;
};
template <char...Cs>
const
std::string static_string<char_sequence<Cs...>>::str = {Cs...};
// helper to get the i_th character (`\0` for out of bound)
template <std::size_t I, std::size_t N>
constexpr char at(const char (&a)[N]) { return I < N ? a[I] : '\0'; }
// helper to check if the c-string will not be truncated
template <std::size_t max_size, std::size_t N>
constexpr bool check_size(const char (&)[N])
{
static_assert(N <= max_size, "string too long");
return N <= max_size;
}
// Helper macros to build char_sequence from c-string
#define PUSH_BACK_8(S, I) \
::push_back<at<(I) + 0>(S)>::push_back<at<(I) + 1>(S)> \
::push_back<at<(I) + 2>(S)>::push_back<at<(I) + 3>(S)> \
::push_back<at<(I) + 4>(S)>::push_back<at<(I) + 5>(S)> \
::push_back<at<(I) + 6>(S)>::push_back<at<(I) + 7>(S)>
#define PUSH_BACK_32(S, I) \
PUSH_BACK_8(S, (I) + 0) PUSH_BACK_8(S, (I) + 8) \
PUSH_BACK_8(S, (I) + 16) PUSH_BACK_8(S, (I) + 24)
#define PUSH_BACK_128(S, I) \
PUSH_BACK_32(S, (I) + 0) PUSH_BACK_32(S, (I) + 32) \
PUSH_BACK_32(S, (I) + 64) PUSH_BACK_32(S, (I) + 96)
// Macro to create char_sequence from c-string (limited to 128 chars) without leading '\0'
#define MAKE_CHAR_SEQUENCE(S) \
strip_sequence<char_sequence<> \
PUSH_BACK_128(S, 0) \
::push_back<check_size<128>(S) ? '\0' : '\0'> \
>::type
// Macro to return an static std::string
#define STATIC_STRING(S) static_string<MAKE_CHAR_SEQUENCE(S)>::str
Live example
gcc has an extension to simplify MAKE_CHAR_SEQUENCE:
template <typename CHAR, CHAR... cs>
constexpr auto operator ""_c() { return char_sequence<cs...>{}; }

Answer 3

If you can use boost 1.55 or greater you can do
#include <boost/utility/string_ref.hpp>
void foo(const boost::string_ref& xyz)
{
}

Answer 4

You could use Boost.Flyweight to make a key-value flyweight from const char* to std::string. I'm not sure about the details, might be that it is enough to use flyweight<std::string> everywhere.

Answer 5

This will work for simple strings - w/o whitespace:
#define DECL_STR(s) const std::string str_##s (#s)
Usage in header (parse once!):
DECL_STR(Foo);
DECL_STR(Bar);
In code:
func(str_Foo);
func(str_Bar);