Writing Quick Code in C++, Quickly

Play Writing Quick Code in C++, Quickly

The Discussion

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    I hope this talk has some commonality with "Writing Fast Code I,II" from C++ and Beyond 2012, because video of these talks were not published here before. Smiley

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    I and II

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    You can find similar video... 3 optimization tips for c++

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    Christian Semmler

    Once again, great talk and insight!


    sizeof... actually has a couple more nice use cases I believe. For example, if you want to generate a type string (to pass through a C interface), you may do it like this

    template<typename... Types>
    struct TypeString {
    static constexpr char value[sizeof...(Types) + 1] = {

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    Elron A Yellin

    You can have the best of both worlds-- pretty api and an option to avoid extra allocations--

    string nextLine(istream&, string&& = string());

    // calling once, or when allocation cost doesn't matter
    auto line = nextLine(strm);

    // calling in a loop
    string line;
    for (...)
    line = nextLine(strm, std::move(line));

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    Alessandro Stamatto

    Nice solution Elron,

    Wouldn't a pass by traditional reference (istream&,string&) be a tiny bit faster than a move in this case? Or is is the same thing?

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    Elron A Yellin

    the default arg ("string()") is a temporary, ie, an rvalue, so the parameter can't be a string&

    Alternatively, we could just write a wrapper/overload, but that's a lot typing and repetition just to save two moves (for the case where we've already decided we don't care about efficiency)
    void nextLine(stream&, string&);
    string nextLine(istream& strm)
    string str;
    nextLine(strm, str);
    return str;

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    Isn't slide 27 supposed to have vtbl[tag][0] and vtbl[tag][1] instead of vtbl[0][tag] and vtbl[1][tag]?

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    Colin McEwan

    Oh dear god.

    From about 16:00 onwards I wanted to kill things.

    Constructing complex C++ template code to hack around the *lack of particular peephole optimisations* in the particular compiler he happens to be using.


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    @Andrea: I think that code is OK but vtbl declaration should be different

    static FP vtbl[totalMethods][totalClasses];

    Multi-dimensional arrays are difficult Wink

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    I agree Colin, he could at least have tried LLVM ;-)

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    @Colin McEwan: I think the point of that exercise was not to do peephole optimization but to provide portable semantics and missing features (like whole-storage assignment). I would not do anything like that myself but for someone who cares about 0.1% performance increase AND is forced to use bit-fields this may be an option.

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    I have used that pattern as well, and noticed that in the case of

    auto line = nextLine(strm);

    it can actually be *faster* than the trivial "string nextLine(istream&)" version as some of the exception handling / stack unwinding code can be moved out of the inner function by the compiler.

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