C++ max function

Sid Kraft 46 Reputation points

Not sure about this one, Virtual Studio:

Works: max({SPANX,SPANY,SPANZ});

Does not work: max(SPANX,SPANY,SPANZ);, generates a "build" error

Why does one need the delimiters ({ )}; ?? Sid Kraft

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2 answers

  1. AI answer

    std::max is overloaded for:

    template<class Type>
constexpr Type& max(const Type& left, const Type& right);

template<class Type>
constexpr Type& max(initializer_list<Type> ilist);

    So there are two relevant forms:

    1. Two arguments: max(a, b)
    2. An initializer_list: max({a, b, c, ...})

    max(SPANX, SPANY, SPANZ); tries to call a function taking three separate parameters, but there is no std::max overload that takes three arguments, so the compiler reports a build error.

    max({SPANX, SPANY, SPANZ}); passes a single argument of type std::initializer_list<T> (constructed from the braced list), which matches the max(initializer_list<Type>) overload. That overload is specifically designed to take an arbitrary number of values and return the largest.

    The braces { ... } are therefore required to form an initializer_list so that the correct overload can be called when using more than two values.

    If only two values are needed, both forms are valid:

    max(SPANX, SPANY); // two-argument overload
max({SPANX, SPANY}); // initializer_list overload

    But for three or more values, the initializer_list form is required.

    References:

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  2. Taki Ly (WICLOUD CORPORATION) 2,145 Reputation points • Microsoft External Staff • Moderator

    Hi @Sid Kraft ,

    The reason you are seeing a build error with max(SPANX, SPANY, SPANZ); but success with max({SPANX, SPANY, SPANZ}); comes down to how the std::max function is defined in the C++ Standard Library (<algorithm>). By default, the standard std::max function is only designed to compare exactly two arguments, or two arguments along with a custom comparison function. It does not have an overload that takes three or more individual template arguments directly.

    So, when you call std::max(SPANX, SPANY, SPANZ);, the C++ compiler gets confused. It assumes you are trying to call this specific overload:

    std::max(const T& a, const T& b, Compare comp)

    It thinks SPANX and SPANY are the two values to compare, and SPANZ is a custom mathematical comparison function. Since SPANZ is just a variable (likely an int) and not a callable function, the build fails.

    Why does {} fix it?

    Starting in C++11, a new overload was added to the standard library that accepts an std::initializer_list. By wrapping your variables in delimited curly braces { }, you are instantiating an std::initializer_list on the fly. You are essentially passing one single argument (a list containing three elements) into std::max, rather than three separate arguments. The function then iterates through that list and correctly returns the largest value.

    Official Documentation Reference:

    You can verify these overloads in the official C++ references for std::max:

    If you look at the C++ Reference link above, you will see the exact signatures:

    1. template< class T > constexpr const T& max( const T& a, const T& b ); (Only compares 2 items)
    2. template< class T, class Compare > constexpr const T& max( const T& a, const T& b, Compare comp ); (Compares 2 items using a custom comparator)
    3. template< class T > constexpr T max( std::initializer_list<T> ilist ); (Compares entirely through a list using {})

    This is why max({SPANX, SPANY, SPANZ}); works. If you found my response helpful or informative, I would greatly appreciate it if you could follow this guide for your confirmation.

    Disclaimer: Some links are non-Microsoft website. The pages appear to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classifies as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.

    Thank you.

    1. Taki Ly (WICLOUD CORPORATION) 2,145 Reputation points • Microsoft External Staff • Moderator

      Hi @Sid Kraft ,

      Have you seen my answer in this post? If you have any question, feel free to reach out. I'm happy to support you.

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