如何从C的“getcwd”返回std :: string功能 [英] How return a std::string from C's "getcwd" function
问题描述
对不起,继续打击这个,但我想学习:)。这是什么好吗?是的,我关心内存泄漏。我找不到一个预先分配char *的方式,因为似乎没有跨平台的方式。
Sorry to keep hammering on this, but I'm trying to learn :). Is this any good? And yes, I care about memory leaks. I can't find a decent way of preallocating the char*, because there simply seems to be no cross-platform way.
const string getcwd()
{
char* a_cwd = getcwd(NULL,0);
string s_cwd(a_cwd);
free(a_cwd);
return s_cwd;
}
UPDATE2:没有Boost或Qt,最常见的东西可以长(请参阅接受的答案)
UPDATE2: without Boost or Qt, the most common stuff can get long-winded (see accepted answer)
推荐答案
如果您想保持标准, getcwd
不需要做任何事情,如果你传递给它一个NULL;你应该在堆栈上分配一个足够大的大多数情况下使用的缓冲区(比如说255个字符),但要准备好让 getcwd
失败 errno == ERANGE
;在这种情况下,你应该分配一个更大的缓冲区,如果必要,增加它的大小。
If you want to remain standard, getcwd
isn't required to do anything if you pass to it a NULL; you should instead allocate on the stack a buffer that is "large enough" for most occasions (say, 255 characters), but be prepared for the occasion in which getcwd
may fail with errno==ERANGE
; in that case you should allocate dinamically a bigger buffer, and increase its size if necessary.
这样的东西可以工作(注意:没有测试,必须改进):
Something like this could work (notice: not tested, just written by scratch, can be surely improved):
string getcwd()
{
const size_t chunkSize=255;
const int maxChunks=10240; // 2550 KiBs of current path are more than enough
char stackBuffer[chunkSize]; // Stack buffer for the "normal" case
if(getcwd(stackBuffer,sizeof(stackBuffer))!=NULL)
return stackBuffer;
if(errno!=ERANGE)
{
// It's not ERANGE, so we don't know how to handle it
throw std::runtime_error("Cannot determine the current path.");
// Of course you may choose a different error reporting method
}
// Ok, the stack buffer isn't long enough; fallback to heap allocation
for(int chunks=2; chunks<maxChunks ; chunks++)
{
// With boost use scoped_ptr; in C++0x, use unique_ptr
// If you want to be less C++ but more efficient you may want to use realloc
std::auto_ptr<char> cwd(new char[chunkSize*chunks]);
if(getcwd(cwd.get(),chunkSize*chunks)!=NULL)
return cwd.get();
if(errno!=ERANGE)
{
// It's not ERANGE, so we don't know how to handle it
throw std::runtime_error("Cannot determine the current path.");
// Of course you may choose a different error reporting method
}
}
throw std::runtime_error("Cannot determine the current path; the path is apparently unreasonably long");
}
顺便说一下,在你的代码中有一个非常错误的事情:你正在尝试dellocate a_cwd(这可能是在非标准扩展,被分配malloc或一些其他内存分配函数,因为getcwd被认为C) delete
:你绝对不应该这样做,请记住,每个分配方法都有它的释放对应,并且它们不能不匹配。
By the way, in your code there's a very wrong thing: you are trying to dellocate a_cwd (which presumably, in the nonstandard extension, is allocated with malloc or with some other memory allocation function, since getcwd is thought for C) with delete
: you absolutely shouldn't do that, keep in mind that each allocation method has its deallocation counterpart, and they must not be mismatched.
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