How to determine if a file can be written using C++

In C++ how can I determine if the program has either read-only access or read-wr开发者_JS百科ite access to a file? I searched the boost filesystem library but I have yet to find something to help me. Right now I thinking of opening the file, trying to write inside and check for error, but that doesn't seem a very appropriate way of doing this.

Any clue?

EDIT : it would need to be cross platform

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At the end of the day, the only way to test if you can write data to a file on a modern OS is to actually try to write it. Lots of things could have happened to the file between tests for permission and the actual write.

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The system call, which most runtime libraries fully support, is

#include <unistd.h>

if (0 == access (char *pathname, int mode))
    // permission is granted

where mode is F_OK to test existence of file, or a mask consisting of the bitwise OR of one or more of R_OK, W_OK, and X_OK.