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Elaborated on PR #1539

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hsutter
2019-11-21 11:33:10 -08:00
parent 39b9ebdf86
commit 54afca9318
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CppCoreGuidelines.md
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@@ -6547,7 +6547,7 @@ That tends to work better than "cleverness" for non-specialists.
The standard C++ mechanism to construct an instance of a type is to call its constructor. As specified in guideline [C.41](#Rc-complete): a constructor should create a fully initialized object. No additional initialization, such as by `memcpy`, should be required. The standard C++ mechanism to construct an instance of a type is to call its constructor. As specified in guideline [C.41](#Rc-complete): a constructor should create a fully initialized object. No additional initialization, such as by `memcpy`, should be required.
A type will provide a copy constructor and/or copy assignment operator to appropriately make a copy of the class, preserving the type's invariants. Using memcpy to copy a non-trivially copyable type has undefined behavior. Frequently this results in slicing, or data corruption. A type will provide a copy constructor and/or copy assignment operator to appropriately make a copy of the class, preserving the type's invariants. Using memcpy to copy a non-trivially copyable type has undefined behavior. Frequently this results in slicing, or data corruption.
##### Example, bad ##### Example, good
struct base struct base
{ {
@@ -6560,16 +6560,28 @@ A type will provide a copy constructor and/or copy assignment operator to approp
void update() override {} void update() override {}
}; };
##### Example, bad
void init(derived& a) void init(derived& a)
{ {
memset(&a, 0, sizeof(derived)); memset(&a, 0, sizeof(derived));
} }
This is type-unsafe and overwrites the vtable.
##### Example, bad
void copy(derived& a, derived& b) void copy(derived& a, derived& b)
{ {
memcpy(&a, &b, sizeof(derived)); memcpy(&a, &b, sizeof(derived));
} }
This is also type-unsafe and overwrites the vtable.
##### Enforcement
- Flag passing a non-trivially-copyable type to `memset` or `memcpy`.
## <a name="SS-containers"></a>C.con: Containers and other resource handles ## <a name="SS-containers"></a>C.con: Containers and other resource handles