Question

在我的C ++代码中经常使用以下类型的辅助函数：

static inline std::string stringf(const char *fmt, ...)
{
    std::string ret;
    // Deal with varargs
    va_list args;
    va_start(args, fmt);
    // Resize our string based on the arguments
    ret.resize(vsnprintf(0, 0, fmt, args));
    // End the varargs and restart because vsnprintf mucked up our args
    va_end(args);
    va_start(args, fmt);
    // Fill the string
    if(!ret.empty())
    {
        vsnprintf(&ret.front(), ret.size() + 1, fmt, args);
    }
    // End of variadic section
    va_end(args);
    // Return the string
    return ret;
}

它有一些好处：

字符串长度没有任意限制
该字符串是就地生成的，不会被复制（如果RVO正常工作）
外界没有惊喜

现在，我遇到了一些问题：

重新扫描varargs的丑陋
事实上，std :: string在内部是一个连续的字符串，后面直接有一个空终结符的空格，但实际上并没有在规范中明确说明。它的含义是 - ＆gt; c_str（）必须是O（1）并返回一个以空字符结尾的字符串，我相信＆amp;（data（）[0]）应该等于＆amp; （*开头（））
vsnprintf（）被调用两次，可能是第一次进行昂贵的丢弃工作

有人知道更好的方法吗？

Answer 1

你结婚了（双关语）std::sprintf()？如果您正在使用C ++和现代std::string，为什么不充分利用新的语言功能并使用可变参数模板来执行返回sprintf的类型安全std::string？

看看这个非常好看的实现：https://github.com/c42f/tinyformat。我想这可以解决你所有的问题。

Answer 2

当您添加标记c++11时，我认为您可以使用它。然后，您可以将代码简化为：

namespace fmt {

template< class ...Args >
std::string sprintf( const char * f, Args && ...args ) {
    int size = snprintf( nullptr, 0, f, args... );
    std::string res;
    res.resize( size );
    snprintf( & res[ 0 ], size + 1, f, args... );
    return res;
}

}

int main() {
    cout << fmt::sprintf( "%s %d %.1f\n", "Hello", 42, 33.22 );
    return 0;
}

http://ideone.com/kSnXKj

Answer 3

不要使用大小为0的第一个$ ./bin/getline_rdfile dat/damages.txt Lines in file: array [ 0] Personal injury damage awards are unliquidated array [ 1] and are not capable of certain measurement; thus, the array [ 2] jury has broad discretion in assessing the amount of array [ 3] damages in a personal injury case. Yet, at the same array [ 4] time, a factual sufficiency review insures that the array [ 5] evidence supports the jury's award; and, although array [ 6] difficult, the law requires appellate courts to conduct array [ 7] factual sufficiency reviews on damage awards in array [ 8] personal injury cases. Thus, while a jury has latitude in array [ 9] assessing intangible damages in personal injury cases, array [ 10] a jury's damage award does not escape the scrutiny of array [ 11] appellate review. array [ 12] array [ 13] Because Texas law applies no physical manifestation array [ 14] rule to restrict wrongful death recoveries, a array [ 15] trial court in a death case is prudent when it chooses array [ 16] to submit the issues of mental anguish and loss of array [ 17] society and companionship. While there is a array [ 18] presumption of mental anguish for the wrongful death array [ 19] beneficiary, the Texas Supreme Court has not indicated array [ 20] that reviewing courts should presume that the mental array [ 21] anguish is sufficient to support a large award. Testimony array [ 22] that proves the beneficiary suffered severe mental array [ 23] anguish or severe grief should be a significant and array [ 24] sometimes determining factor in a factual sufficiency array [ 25] analysis of large non-pecuniary damage awards.。而是使用具有一些可能大小的堆栈缓冲区（例如64到4096之间的东西），如果适合，则将其复制到返回值中。

您对$ valgrind ./bin/getline_rdfile dat/damages.txt ==14321== Memcheck, a memory error detector ==14321== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. ==14321== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info ==14321== Command: ./bin/getline_rdfile dat/damages.txt ==14321== Lines in file: array [ 0] Personal injury damage awards are unliquidated <snip> ... array [ 25] analysis of large non-pecuniary damage awards. ==14321== ==14321== HEAP SUMMARY: ==14321== in use at exit: 0 bytes in 0 blocks ==14321== total heap usage: 29 allocs, 29 frees, 3,997 bytes allocated ==14321== ==14321== All heap blocks were freed -- no leaks are possible ==14321== ==14321== For counts of detected and suppressed errors, rerun with: -v ==14321== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)邻居的担忧是错误的。它是明确的，完全可以依赖。

最后 - 我想回顾一下编译时格式检查库会更好。你需要C ++ 14才能获得全部功能，但是C ++ 11支持足够多，你应该能够vsnprintf一些头代码而不会丢失任何表达。请记住考虑std::string！

在C ++中有没有更优雅的方式将sprintf和std :: string结合起来？

3 个答案: