[英]Precision loss when passing arguments using va_list and printf
I have a logging function which works like this: 我有一个日志功能,其工作方式如下:
// public function:
void ConsoleUI::log(const std::string& format, ...) {
va_list args;
va_start(args, format);
log(format, args);
va_end(args);
}
// overloaded private function:
void ConsoleUI::log(const std::string& format, va_list args) {
//wprintw( outWin, format.c_str(), args);
//wprintw( outWin, "\n");
printf( format.c_str(), args);
printf( "\n");
}
(Side note: This should do the exact same thing as just a normal printf, so it's pretty useless in this state. That's because this is a minumal working example. In the end, this should work with ncurses - see commented section.) (旁注:这应该与普通的printf完全相同,因此在这种状态下它是无用的。这是因为这是一个极小的工作示例。最后,这应该与ncurses一起使用 - 请参阅注释部分。)
I then create an instance of the ConsoleUI class, called ui: 然后我创建一个名为ui的ConsoleUI类的实例:
ConsoleUI ui;
And later I use it to log stuff, more precisely, a time_duration in microseconds: 后来我用它来记录东西,更确切地说,是一个以微秒为单位的time_duration:
now = boost::posix_time::microsec_clock::local_time();
boost::posix_time::time_duration delta = now - lastTime;
double dt = 1e-6*(double)delta.total_microseconds();
lastTime = now;
ui.log( "logged dt: %f", dt );
ui.log( "logged dt 2: %lu", delta.total_microseconds());
printf( "dt: %f\n", dt);
printf( "dt 2: %lu\n", delta.total_microseconds());
The output I get: 我得到的输出:
logged dt: 0.000000
logged dt 2: 140736013247624
dt: 0.018739
dt 2: 18739
What I expected: 我的期望:
logged dt: 0.018739
logged dt 2: 18739
dt: 0.018739
dt 2: 18739
Note that over multiple calls of this, the values in the last two lines change slightly (as is to be expected of a delta time) and the first two values don't change - which looks like there's something wrong with the format. 请注意,通过多次调用,最后两行中的值稍有变化(如预期的增量时间),前两个值不会改变 - 这看起来格式有问题。
So, bottom line: Calling printf directly works, but passing it down the logger and then calling printf doesn't work... 所以,底线:调用printf直接工作,但将其传递给记录器,然后调用printf不起作用...
Since you can't actually pass your arguments from log
to printf
, you are passing a va_list
- this is the right solution, but you can't pass a va_list
to printf. 由于您实际上无法将参数从log
传递给printf
,因此您传递的是va_list
- 这是正确的解决方案,但您无法将va_list
传递给printf。
This line: 这一行:
printf( format.c_str(), args);
prints args
with the format in format
. 打印args
与格式format
。 Given that args
is of the type va_list
(typically implemented in the comiler ABI as a void pointer) it will not represent the data that you expect in the format string. 假设args
的类型为va_list
(通常在编译器ABI中作为void指针实现),它将不代表您在格式字符串中所期望的数据。 If you use 如果你使用
vprintf( format.c_str(), args);
it should work just fine, that's what vprintf
is for. 它应该工作得很好,这就是vprintf
的用途。
(Note that it's nothing to do with "loss of precision" - it's passing incorrect type of argument to printf - and since it's a programmaticly formulated string, even enabling warnings is not going to work). (请注意,它与“精度损失”无关 - 它将不正确的参数类型传递给printf - 并且因为它是一个程序化的字符串,即使启用警告也不会起作用)。
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