I want in my program read double values from different text files. Some files contain doubles in format 'xx.yyy' (dot is a separator), but some other - 'xx,yyy' (comma is a separator). I use c++ streams for reading.
If you know the format of each file ahead of time, you can imbue() a std::ifstream with a custom std::locale whose std::numpunct facet specifies the desired thousands_sep and decimal_point characters. Then operator>> will be able to parse the double values as expected.
See How can I set the decimal separator to be a comma? for how to specify a custom decimal_point. The same approach applies to thousands_sep as well.
If you don't know the format ahead of time, you will have to read the doubles as strings first, then manually parse them to determine which format they are using, and then you can imbue() a std::istringstream as needed to then convert the strings to double. If all of the doubles in a given file use the same format, then you could do this for the 1st double and then imbue() the std::ifstream as needed for subsequent doubles.