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1 # base::Optional | 1 # base::Optional |
2 | 2 |
3 `base::Optional<T>` is a container that might contain an instance of `T`. | 3 `base::Optional<T>` is a container that might contain an instance of `T`. |
4 | 4 |
5 [TOC] | 5 [TOC] |
6 | 6 |
7 ## History | 7 ## History |
8 | 8 |
9 [base::Optional<T>](https://code.google.com/p/chromium/codesearch#chromium/src/b
ase/optional.h) | 9 [base::Optional<T>](https://code.google.com/p/chromium/codesearch#chromium/src/b
ase/optional.h) |
10 is an implementation of [std::optional<T>](http://en.cppreference.com/w/cpp/util
ity/optional), | 10 is an implementation of [std::optional<T>](http://en.cppreference.com/w/cpp/util
ity/optional), |
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94 It can also be used for simple types, for example when a structure wants to | 94 It can also be used for simple types, for example when a structure wants to |
95 represent whether the user or the underlying data structure has some value | 95 represent whether the user or the underlying data structure has some value |
96 unspecified, a `base::Optional<int>` would be easier to understand than a | 96 unspecified, a `base::Optional<int>` would be easier to understand than a |
97 special value representing the lack of it. For example, using -1 as the | 97 special value representing the lack of it. For example, using -1 as the |
98 undefined value when the expected value can't be negative. | 98 undefined value when the expected value can't be negative. |
99 | 99 |
100 ## When not to use? | 100 ## When not to use? |
101 | 101 |
102 It is recommended to not use `base::Optional<T>` as a function parameter as it | 102 It is recommended to not use `base::Optional<T>` as a function parameter as it |
103 will force the callers to use `base::Optional<T>`. Instead, it is recommended to | 103 will force the callers to use `base::Optional<T>`. Instead, it is recommended to |
104 keep using `T*` for arguments that can be ommited, with `nullptr` representing | 104 keep using `T*` for arguments that can be omitted, with `nullptr` representing |
105 no value. | 105 no value. |
106 | 106 |
107 Furthermore, depending on `T`, MSVC might fail to compile code using | 107 Furthermore, depending on `T`, MSVC might fail to compile code using |
108 `base::Optional<T>` as a parameter because of memory alignment issues. | 108 `base::Optional<T>` as a parameter because of memory alignment issues. |
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