我必须用sprintf格式化std::string,并将其发送到文件流。我该怎么做呢?


当前回答

到目前为止,所有的答案似乎都有一个或多个这样的问题:(1)它可能无法在vc++上工作(2)它需要额外的依赖,如boost或fmt(3)它太复杂的自定义实现,可能没有经过很好的测试。

下面的代码解决了上述所有问题。

#include <string>
#include <cstdarg>
#include <memory>

std::string stringf(const char* format, ...)
{
    va_list args;
    va_start(args, format);
    #ifndef _MSC_VER

        //GCC generates warning for valid use of snprintf to get
        //size of result string. We suppress warning with below macro.
        #ifdef __GNUC__
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wformat-nonliteral"
        #endif

        size_t size = std::snprintf(nullptr, 0, format, args) + 1; // Extra space for '\0'

        #ifdef __GNUC__
        # pragma GCC diagnostic pop
        #endif

        std::unique_ptr<char[]> buf(new char[ size ] ); 
        std::vsnprintf(buf.get(), size, format, args);
        return std::string(buf.get(), buf.get() + size - 1 ); // We don't want the '\0' inside
    #else
        int size = _vscprintf(format, args);
        std::string result(++size, 0);
        vsnprintf_s((char*)result.data(), size, _TRUNCATE, format, args);
        return result;
    #endif
    va_end(args);
}    

int main() {
    float f = 3.f;
    int i = 5;
    std::string s = "hello!";
    auto rs = stringf("i=%d, f=%f, s=%s", i, f, s.c_str());
    printf("%s", rs.c_str());
    return 0;
}

注:

Separate VC++ code branch is necessary because VC++ has decided to deprecate snprintf which will generate compiler warnings for other highly voted answers above. As I always run in "warnings as errors" mode, its no go for me. The function accepts char * instead of std::string. This because most of the time this function would be called with literal string which is indeed char *, not std::string. In case you do have std::string as format parameter, then just call .c_str(). Name of the function is stringf instead of things like string_format to keepup with printf, scanf etc. It doesn't address safety issue (i.e. bad parameters can potentially cause seg fault instead of exception). If you need this then you are better off with boost or fmt libraries. My preference here would be fmt because it is just one header and source file to drop in the project while having less weird formatting syntax than boost. However both are non-compatible with printf format strings so below is still useful in that case. The stringf code passes through GCC strict mode compilation. This requires extra #pragma macros to suppress false positives in GCC warnings.

以上代码已在,

GCC 4.9.2 11 / c++ / C + + 14 vc++编译器19.0 铿锵声3.7.0

其他回答

我喜欢的一个解决方案是,在使缓冲区足够大之后,用sprintf直接在std::string缓冲区中执行此操作:

#include <string>
#include <iostream>

using namespace std;

string l_output;
l_output.resize(100);

for (int i = 0; i < 1000; ++i)
{       
    memset (&l_output[0], 0, 100);
    sprintf (&l_output[0], "\r%i\0", i);

    cout << l_output;
    cout.flush();
}

因此,创建std::string,调整它的大小,直接访问它的缓冲区…

如果你只想要一个类似printf的语法(不需要自己调用printf),可以看看Boost Format。

下面是@iFreilicht答案的稍微修改版本,更新到c++ 14(使用make_unique函数而不是原始声明),并增加了对std::string参数的支持(基于Kenny Kerr的文章)

#include <iostream>
#include <memory>
#include <string>
#include <cstdio>

template <typename T>
T process_arg(T value) noexcept
{
    return value;
}

template <typename T>
T const * process_arg(std::basic_string<T> const & value) noexcept
{
    return value.c_str();
}

template<typename ... Args>
std::string string_format(const std::string& format, Args const & ... args)
{
    const auto fmt = format.c_str();
    const size_t size = std::snprintf(nullptr, 0, fmt, process_arg(args) ...) + 1;
    auto buf = std::make_unique<char[]>(size);
    std::snprintf(buf.get(), size, fmt, process_arg(args) ...);
    auto res = std::string(buf.get(), buf.get() + size - 1);
    return res;
}

int main()
{
    int i = 3;
    float f = 5.f;
    char* s0 = "hello";
    std::string s1 = "world";
    std::cout << string_format("i=%d, f=%f, s=%s %s", i, f, s0, s1) << "\n";
}

输出:

i = 3, f = 5.000000, s = hello world

如果需要,可以随意将这个答案与原始答案合并。

c++ 17解决方案(这将工作于std::string和std::wstring):

分配一个缓冲区,格式化它,然后复制到另一个字符串是不高效的。可以创建格式化字符串大小的std::string,并直接格式化到字符串缓冲区中:

#include <string>
#include <stdexcept>
#include <cwchar>
#include <cstdio>
#include <type_traits>

template<typename T, typename ... Args>
std::basic_string<T> string_format(T const* const format, Args ... args)
{
    int size_signed{ 0 };

    // 1) Determine size with error handling:    
    if constexpr (std::is_same_v<T, char>) { // C++17
        size_signed = std::snprintf(nullptr, 0, format, args ...);
    }
    else {
        size_signed = std::swprintf(nullptr, 0, format, args ...);
    }  
    if (size_signed <= 0) {
        throw std::runtime_error("error during formatting.");
    }
    const auto size = static_cast<size_t>(size_signed);

    // 2) Prepare formatted string:
    std::basic_string<T> formatted(size, T{});
    if constexpr (std::is_same_v<T, char>) { // C++17
        std::snprintf(formatted.data(), size + 1, format, args ...); // +1 for the '\0' (it will not be part of formatted).
    }
    else {
        std::swprintf(formatted.data(), size + 1, format, args ...); // +1 for the '\0' (it will not be part of formatted).
    }

    return formatted; // Named Return Value Optimization (NRVO), avoids an unnecessary copy. 
}

此外:通常,format参数是char[] / wchar_t[] &创建std::string对象效率不高。传递char*或wchar_t* &如果你已经有一个std::string对象,你仍然可以使用它作为your_string.c_str()。例子:

int main()
{
    int i{ 0 };

    // The format parameter is a char[] / wchar_t[]:

    const std::string title1 = string_format("story[%d].", ++i); // => "story[1]"

    const std::wstring title2 = string_format(L"story[%d].", ++i); // => L"story[2]"

    // If you already have a std::string object:

    const std::string format1{ "story[%d]." };
    const std::string title3 = string_format(format1.c_str(), ++i); // => "story[3]"

    const std::wstring format2{ L"story[%d]." };
    const std::wstring title4 = string_format(format2.c_str(), ++i); // => L"story[4]"  
}

这是一个特定于Windows的解决方案,旨在避免Visual Studio中的编译器警告而不消除它们。所讨论的警告是针对使用std::string和va_start,这会错误地产生警告,以及针对使用已弃用的printf变量。

template<typename ... va>
std::string Format( const std::string& format, va ... args )
{
    std::string s;
    s.resize( _scprintf( format.c_str(), args ... ) + 1 );
    s.resize( _snprintf_s( s.data(), s.capacity(), _TRUNCATE, format.c_str(), args ... ) );
    return s;
}

template<typename ... va>
std::wstring Format( const std::wstring& format, va ... args )
{
    std::wstring s;
    s.resize( _scwprintf( format.c_str(), args ... ) + 1 );
    s.resize( _snwprintf_s( s.data(), s.capacity(), _TRUNCATE, format.c_str(), args ... ) );
    return s;
}

std::string s = Format( "%hs %d", "abc", 123 );
std::wstring ws = Format( L"%hs %d", "abc", 123 );