假设我需要调用一个函数foo
,该函数std::string
从代码中的很多地方获取const引用:
int foo(const std::string&);
..
foo("bar");
..
foo("baz");
用这样的字符串文字调用函数将创建临时std::string
对象,每次都复制文字。
除非我没有记错,否则编译器不会通过为std::string
每个文字创建一个静态对象来优化此效果,该对象可用于后续调用。我知道g ++具有高级的字符串池机制,但是我认为它不会扩展到std::string
对象本身。
我可以自己进行“优化”,这会使代码的可读性降低:
static std::string bar_string("bar");
foo(bar_string);
..
static std::string baz_string("baz");
foo(baz_string);
使用Callgrind,我可以确认这确实可以提高程序速度。
我以为我会尝试为此创建一个宏,但我不知道是否可能。我想要的是这样的:
foo(STATIC_STRING("bar"));
..
foo(STATIC_STRING("baz"));
我尝试使用文字作为模板参数创建模板,但是事实证明这是不可能的。而且由于不可能在代码块中定义函数,所以我全都没主意。
有没有做到这一点的优雅方法,还是我不得不求助于可读性较低的解决方案?
您可以使用类似的方法来创建static std::string
“就地”:
#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
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