* common/file_util: Make IOFile's WriteString take a std::string_view
We don't need to force the usage of a std::string here, and can instead
use a std::string_view, which allows writing out other forms of strings
(e.g. C-style strings) without any unnecessary heap allocations.
* common/file_util: Remove unnecessary c_str() calls
The file stream open functions have supported std::string overloads
since C++11, so we don't need to use c_str() here. Same behavior, less
code.
* common/file_util: Make ReadFileToString and WriteStringToFile consistent
Makes the parameter ordering consistent, and also makes the filename
parameter a std::string. A std::string would be constructed anyways with
the previous code, as IOFile's only constructor with a filepath is one
taking a std::string.
We can also make WriteStringToFile's string parameter utilize a
std::string_view for the string, making use of our previous changes to
IOFile.
* common/file_util: Remove duplicated documentation comments
These are already present within the header, so they don't need to be
repeated in the cpp file.
* common/file_util: Make GetCurrentDir() return a std::optional
nullptr was being returned in the error case, which, at a glance may
seem perfectly OK... until you realize that std::string has the
invariant that it may not be constructed from a null pointer. This
means that if this error case was ever hit, then the application would
most likely crash from a thrown exception in std::string's constructor.
Instead, we can change the function to return an optional value,
indicating if a failure occurred.
* common/file_util: Remove unnecessary return at end of void StripTailDirSlashes()
While we're at it, also invert the conditional into a guard clause.
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