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std::optional does not support references (std::optional<MyVeryLargeType&>) however unlike boost, so this precludes usage with non-copyable types.

paulddraper 7y ago
That was decided against for now, though I'm not sure why. [1] In any case, you can use std::optional<std::reference_wrapper<MyVeryLargeType>>. [1] https://stackoverflow.com/a/26895581 https://stackoverflow.com/a/26895581
- jcelerier 7y ago
  > In any case, you can use std::optional<std::reference_wrapper<MyVeryLargeType>>. I would frankly give a negative code review to anyone who would do that. You go from e.g. void my_function(T* foo, T* bar, int baz) { // ... foo->stuff(bar, baz); } to void my_function(std::optional<std::reference_wrapper<T>> foo, std::optional<std::reference_wrapper<T>> bar, int baz) { // ... foo->get().call(&bar->get(), z); } this also has more overhead (sizeof(std::optional<std::reference_wrapper<T>>) is twice the size of T* ), and let's not even start talking about calling conventions and the compile time cost of having to include both <functional> and <optional> everywhere.
  - paulddraper 7y ago
    It's hard to tell, but if T* foo can be NULL (and I assume that it can, though there's no visible indication), your first function is gonna segfault. It's easy to tell that in the second function. Second function makes for a better code review.
    - jcelerier 7y ago
      heh, to say that I originally wrote if(foo) and then replaced it by //... because that was not the point. Also, dereferencing an unset optional is also UB, so it would segfault all the same given the same preconditions.
      - paulddraper 7y ago
        Yes. The code is equivalent, but it's clear that the second is meant to optional. The first has no such indication, except hopefully some human-readable comment that the pointer may be NULL.
        jcelerier 7y ago
        > The first has no such indication, except hopefully some human-readable comment that the pointer may be NULL. I don't understand. If the parameter was not meant to be sometimes null it would be a reference; not a pointer. The fact that a pointer is used is the "this may be null" indication.

paulddraper 7y ago

That was decided against for now, though I'm not sure why. [1] In any case, you can use std::optional<std::reference_wrapper<MyVeryLargeType>>. [1] https://stackoverflow.com/a/26895581 https://stackoverflow.com/a/26895581

jcelerier 7y ago
> In any case, you can use std::optional<std::reference_wrapper<MyVeryLargeType>>. I would frankly give a negative code review to anyone who would do that. You go from e.g. void my_function(T* foo, T* bar, int baz) { // ... foo->stuff(bar, baz); } to void my_function(std::optional<std::reference_wrapper<T>> foo, std::optional<std::reference_wrapper<T>> bar, int baz) { // ... foo->get().call(&bar->get(), z); } this also has more overhead (sizeof(std::optional<std::reference_wrapper<T>>) is twice the size of T* ), and let's not even start talking about calling conventions and the compile time cost of having to include both <functional> and <optional> everywhere.
- paulddraper 7y ago
  It's hard to tell, but if T* foo can be NULL (and I assume that it can, though there's no visible indication), your first function is gonna segfault. It's easy to tell that in the second function. Second function makes for a better code review.
  - jcelerier 7y ago
    heh, to say that I originally wrote if(foo) and then replaced it by //... because that was not the point. Also, dereferencing an unset optional is also UB, so it would segfault all the same given the same preconditions.
    - paulddraper 7y ago
      Yes. The code is equivalent, but it's clear that the second is meant to optional. The first has no such indication, except hopefully some human-readable comment that the pointer may be NULL.
      - jcelerier 7y ago
        > The first has no such indication, except hopefully some human-readable comment that the pointer may be NULL. I don't understand. If the parameter was not meant to be sometimes null it would be a reference; not a pointer. The fact that a pointer is used is the "this may be null" indication.