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

String没有你需要的东西,但是std::stringstream有。使用stringstream创建字符串,然后提取字符串。这里有一个关于你可以做的事情的全面列表。例如:

cout.setprecision(10); //stringstream is a stream like cout

将在打印双精度或浮点数时提供10位小数点后的精度。

其他回答

你可以试试这个:

string str;
str.resize( _MAX_PATH );

sprintf( &str[0], "%s %s", "hello", "world" );
// optionals
// sprintf_s( &str[0], str.length(), "%s %s", "hello", "world" ); // Microsoft
// #include <stdio.h>
// snprintf( &str[0], str.length(), "%s %s", "hello", "world" ); // c++11

str.resize( strlen( str.data() ) + 1 );

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。

可以使用iomanip头文件格式化cout中的c++输出。 在使用类似的任何helper函数之前,请确保包含iomanip头文件 Setprecision, setfill等等。

下面是我过去用来在向量中打印平均等待时间的代码片段,这是我“累积”的。

#include<iomanip>
#include<iostream>
#include<vector>
#include<numeric>

...

cout<< "Average waiting times for tasks is " << setprecision(4) << accumulate(all(waitingTimes), 0)/double(waitingTimes.size()) ;
cout << " and " << Q.size() << " tasks remaining" << endl;

下面是如何格式化c++流的简要描述。 http://www.cprogramming.com/tutorial/iomanip.html

从Dacav和pixelpoint的答案中获得灵感。我玩了一下,得到了这个:

#include <cstdarg>
#include <cstdio>
#include <string>

std::string format(const char* fmt, ...)
{
    va_list vl;

    va_start(vl, fmt);
    int size = vsnprintf(0, 0, fmt, vl) + sizeof('\0');
    va_end(vl);

    char buffer[size];

    va_start(vl, fmt);
    size = vsnprintf(buffer, size, fmt, vl);
    va_end(vl);

    return std::string(buffer, size);
}

通过合理的编程实践,我相信代码应该足够了,但是我仍然对更安全的替代方案持开放态度,这些替代方案仍然足够简单,不需要c++ 11。


下面是另一个版本,它使用初始缓冲区来防止在初始缓冲区已经足够多时再次调用vsnprintf()。

std::string format(const char* fmt, ...)
{

    va_list vl;
    int size;

    enum { INITIAL_BUFFER_SIZE = 512 };

    {
        char buffer[INITIAL_BUFFER_SIZE];

        va_start(vl, fmt);
        size = vsnprintf(buffer, INITIAL_BUFFER_SIZE, fmt, vl);
        va_end(vl);

        if (size < INITIAL_BUFFER_SIZE)
            return std::string(buffer, size);
    }

    size += sizeof('\0');

    char buffer[size];

    va_start(vl, fmt);
    size = vsnprintf(buffer, size, fmt, vl);
    va_end(vl);

    return std::string(buffer, size);
}

(事实证明,这个版本与Piti Ongmongkolkul的答案相似,只是它没有使用new和delete[],并且在创建std::string时指定了大小。

The idea here of not using new and delete[] is to imply usage of the stack over the heap since it doesn't need to call allocation and deallocation functions, however if not properly used, it could be dangerous to buffer overflows in some (perhaps old, or perhaps just vulnerable) systems. If this is a concern, I highly suggest using new and delete[] instead. Note that the only concern here is about the allocations as vsnprintf() is already called with limits, so specifying a limit based on the size allocated on the second buffer would also prevent those.)

这个问题已经解决了。但是,我认为这是c++中格式化字符串的另一种方式

class string_format {
private:
    std::string _result;
public:
    string_format( ) { }
    ~string_format( ) { std::string( ).swap( _result ); }
    const std::string& get_data( ) const { return _result; }
    template<typename T, typename... Targs>
    void format( const char* fmt, T value, Targs... Fargs ) {
        for ( ; *fmt != '\0'; fmt++ ) {
            if ( *fmt == '%' ) {
                _result += value;
                this->format( fmt + 1, Fargs..., 0 ); // recursive call
                return;
            }
            _result += *fmt;
        }
    }
    friend std::ostream& operator<<( std::ostream& ostream, const string_format& inst );
};
inline std::string& operator+=( std::string& str, int val ) {
    str.append( std::to_string( val ) );
    return str;
}
inline std::string& operator+=( std::string& str, double val ) {
    str.append( std::to_string( val ) );
    return str;
}
inline std::string& operator+=( std::string& str, bool val ) {
    str.append( val ? "true" : "false" );
    return str;
}
inline std::ostream& operator<<( std::ostream& ostream, const string_format& inst ) {
    ostream << inst.get_data( );
    return ostream;
}

并测试这个类:

string_format fmt;
fmt.format( "Hello % and is working ? Ans: %", "world", true );
std::cout << fmt;

你可以在这里查一下