Compute-intensive cross-platform library, which language?
Suppose you were going to create a compute-intensive cross-platform library in 2024. What would you use? C, C++, Rust, or something else?
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If the domain is scientific computing, consider Fortran, for which compilers are available on most platforms.
It depends how portable they want their code to be across implementations and over time. Fortran compilers vary widely in their interpretations of the language, more so than mainstream C/C++ compilers do.