c#中格式化字符串很简单，比如我们可以这样格式化一个字符串:
string str = string.format("test {0}, {1}, {2}, {1}, {0} sample", 1, 2.3, "ok"); Console.WriteLine(str);
将输出：test 1, 2.3, ok, 2.3, 1 sample
这个格式化方法用起来很简单，支持基本类型的参数，比如int、double和string等，用起来很方便。遗憾的是c++中目前还没有类似的格式化方法。boost库提供了一个format方法，但用起来没有c#的format方法简单和灵活。让我们来看看boost.format如何实现上面的格式化：
string str = boost::format("test %1%, %2%, %3%, %4%, %5% sample")%1%2.3%"ok"%2.3%1; cout<<str<<endl;
boost::format的问题是需要写很多%，用起来繁琐又不直观。c++还缺少一个类似于c#的format方法，要实现一个类似的简单的format也不难，我将实现一个简单的format，基本用法和c#一致，为了保持简单，不支持复杂的功能，只支持基本类型的转换，转换的格式控制就不支持了，简单够用就好。下面来看看format的具体实现吧：
#include <tuple> #include <type_traits> #include <string>
#include<cctype> using namespace std; #include "Variant.hpp" namespace detail { using Value = Variant<uint8_t, uint16_t, uint32_t, uint64_t, int8_t, int16_t, int32_t, int64_t, float, double, string, char*, const char*>; char g_buf[2000] = {}; template<size_t k, typename Tuple, typename F> typename std::enable_if < (k == std::tuple_size<Tuple>::value)>::type GetArgByIndex(size_t, Tuple&, F&, char*&) { throw std::invalid_argument("arg index out of range"); } template<size_t k = 0, typename Tuple, typename F> typename std::enable_if < (k < std::tuple_size<Tuple>::value)>::type GetArgByIndex(size_t index, Tuple& tp, F& f, char*& p) { if (k == index) { f(p, std::get<k>(tp)); } else { GetArgByIndex<k + 1>(index, tp, f, p); } } inline int GetIndex(char*& p) { char temp[3] = {}; int i = 0; while (*p != '}'&&*p != ' ') { if (i >= 2) throw std::invalid_argument("index is out of range."); if (std::isdigit(*p)) { //push digit temp[i++] = *p; char next = *(p + 1); if (std::isdigit(next)) { temp[i++] = next; p += 2; continue; } //validate arg if (!std::isspace(next) && next != '}') { throw std::invalid_argument("invalid argument."); } } p++; } return i == 0 ? -1 : std::atoi(temp); } inline void Fun(char*& buf, Value t) { t.Visit([&buf](int i) { _itoa(i, buf, 10);
buf += strlen(buf); }, [&buf](double i) { auto r = sprintf(buf, "%.6f", i); buf += r; }, [&buf](int64_t i) { auto r = sprintf(buf, "%"PRId64, i); buf += r; }, [&buf](uint64_t i) { auto r = sprintf(buf, "%"PRIu64, i); buf += r; }, [&buf](const char* p) { int len = strlen(p); memcpy(buf, p, len); buf += len; }, [&buf](string& s) { memcpy(buf, s.data(), s.size()); buf += s.size(); }); } } template<typename... Args> inline string format(string& str, Args... args) { using namespace detail; char* buf = g_buf; auto tp = std::tuple<Args...>(args...); char* p = (char*) str.c_str(); char* original = p; int len = str.size() + 1; int last = 0; while (true) { if (*p == '{') { //copy content befor { last = p - original; memcpy(buf, original, last); buf += last; //format args int index = GetIndex(p); if (index >= 0) { GetArgByIndex<0>(index, tp, Fun, buf); } //skip } original = p + 1; }
else if(*p==' ')
{
last = p - original;
memcpy(buf, original, last);
break;
} p++; } string s = g_buf; memset(g_buf, 0, buf - g_buf); return s; }
再来看看测试代码：
string str = "it is { 0 }, and {01}, {2}, {01}, {1}"; cout<<format(str, 11, 2.1, "tt")<<endl;
将输出：it is 11, and 2.1, tt, 2.1 1
用法和c#的一致，比boost的format用起来更方便。这个format支持最多100个参数（0-99），格式化的最长的字符串为2K。
/**********************更新，做简化和修改bug*******************************/
#include <tuple> #include <type_traits> #include <string> #include <inttypes.h>
#include <cctype> using namespace std; namespace detail { char g_buf[2000] = {}; inline void FormatArg(char*& buf, int i) { _itoa(i, buf, 10); buf += strlen(buf); } inline void FormatArg(char*& buf, double i) { auto r = sprintf(buf, "%.6f", i); buf += r; } inline void FormatArg(char*& buf, int64_t i) { auto r = sprintf(buf, "%"PRId64, i); buf += r; } inline void FormatArg(char*& buf, uint64_t i) { auto r = sprintf(buf, "%"PRIu64, i); buf += r; } inline void FormatArg(char*& buf, const char* p) { int len = strlen(p); memcpy(buf, p, len); buf += len; } inline void FormatArg(char*& buf, string& s) { memcpy(buf, s.data(), s.size()); buf += s.size(); } template<size_t k, typename Tuple> typename std::enable_if < (k == std::tuple_size<Tuple>::value)>::type inline GetArgByIndex(size_t, Tuple&, char*&) { throw std::invalid_argument("arg index out of range"); } template<size_t k = 0, typename Tuple> typename std::enable_if < (k < std::tuple_size<Tuple>::value)>::type inline GetArgByIndex(size_t index, Tuple& tp, char*& p) { if (k == index) { FormatArg(p, std::get<k>(tp)); } else { GetArgByIndex<k + 1>(index, tp, p); } } inline int GetIndex(char*& p) { char temp[3] = {}; int i = 0; while (*p != '}'&&*p != ' ') { if (i >= 2) throw std::invalid_argument("index is out of range."); if (std::isdigit(*p)) { //push digit temp[i++] = *p; char next = *(p + 1); if (std::isdigit(next)) { temp[i++] = next; p += 2; continue; } //validate arg if (!std::isspace(next) && next != '}') { throw std::invalid_argument("invalid argument."); } } p++; } return i == 0 ? -1 : std::atoi(temp); } } template<typename... Args> inline string format(const string& src, Args... args) { return format((char*) src.c_str(), args...); } template<typename... Args> inline string format(char* src, Args... args) { using namespace detail; char* buf = g_buf; auto tp = std::tuple<Args...>(args...); char* p = src; char* original = p; int len = strlen(src) + 1; int last = 0;
while (true)
{
if (*p == '{')
{
//copy content befor {
last = p - original;
memcpy(buf, original, last);
buf += last;
//format args
int index = GetIndex(p);
if (index >= 0)
{
GetArgByIndex<0>(index, tp, Fun, buf);
}
//skip }
original = p + 1;
}
else if(*p==' ')
{
last = p - original;
memcpy(buf, original, last);
break;
}
p++;
}
string s = g_buf;
memset(g_buf, 0, buf - g_buf);
return s;
}
简化代码的实现，移除对Variant的依赖，直接通过重载函数来格式化参数。经简单测试效率比boost::format要高六倍以上。
如果你发现还有更好的format请告诉我，如果没有请点一下推荐，谢谢。^_^
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