In my code, I want to identify some properties about the contents of a file, before deciding how to read the file. (That is, I search for a keyword, if found, it's going to be read with foo(std::ifstream&)
, else with bar(std::ifstream&)
).
I implemented the method that searches for the keyword as
bool containsKeyword(std::ifstream& file, const char* keyword)
{
for ( std::string line; std::getline(file, line); )
{
if ( line == keyword )
{
return true;
}
}
return false;
}
This modifies the position of the file stream (either the end, if the keyword isn't found, or the position of the keyword). However I want that the position is reset after the search. This can be done with a ScopeGuard:
class FilePositionScopeGuard
{
private:
std::ifstream& file;
using FilePosition = decltype(std::declval<std::ifstream>().tellg());
FilePosition initial_position;
public:
FilePositionScopeGuard(std::ifstream& file_)
:
file(file_),
initial_position(file.tellg())
{
}
~FilePositionScopeGuard()
{
file.clear();
file.seekg(initial_position);
}
};
Now we add this to the method:
bool containsKeyword(std::ifstream& file, const char* keyword)
{
FilePositionScopeGuard guard(file);
for ( std::string line; std::getline(file, line); )
{
...
That's nice, because with exactly one additional line in the method, we get the behaviour of not modifying the std::ifstream
no matter how the method is exited (one of the returns or an exception).
However, the method bool containsKeyword(std::ifstream&, const char*);
does not express the constness. How can I adjust my method to express (at the level of the interface) that the method will not alter the current state?