C++ Enum storage size conversion prevention

Question

Is is possible to prevent explicit enum storage size conversions? I tried overloading = but this does not seem to work on primitive types.

enum GlobalSettingTableType : std::uint16_t
{
   first = 0,
   second = 1,
   other = 1000 // out of range for uint8_t
};


GlobalSettingTableType test = GlobalSettingTableType::other;
std::uint16_t copyOK = test;
std::uint8_t copyfail = test; // Should not compile

overflow is well defined for unsigned types. You could use an enum class, and then write and constrain the operations from there. — NathanOliver
– NathanOliver, Commented Aug 7 at 16:22
A enum class wouldn't allow implicit conversions at all. If you need more control than this, make a class. — HolyBlackCat
– HolyBlackCat, Commented Aug 7 at 16:28
I don't know why it's not warning (directly assigning an uint16_t to an `` uint8_t` gives the expected warning about a possible value change); however, if you were to switch to enum class -- which is recommended anyway for type-safety -- you wouldn't even be able to assign to other types without a cast. — Christian Stieber
– Christian Stieber, Commented Aug 7 at 16:32
This is what brace initialization is for use : std::uint8_t copyfail{test}; — Pepijn Kramer
– Pepijn Kramer, Commented Aug 7 at 17:06

Marek R · Accepted Answer · 2025-08-07 22:42:53Z

5

Depending on compiler you are using you can tweak warnings to file an error if there is mismatch between emum and target integer type.

MSVC simple /W4 covers this scenario - it enables C4244.
clang requires adding -Wconversion (standard set -Wall -Wextra -pedantic is not enogh)
gcc has some flags (for example -Wenum-int-mismatch), but they are working only for C and Objective-C only (I do not know why). I've failed to find alternative gcc flag which would work for C++.

Here is godbolt demo

In case you are writing a new code, using braces as coding convention is a good solution for all compilers - as Pepijn Kramer proposed in comment.
Tweaking warnings should be good for already existing code.

edited Aug 7 at 22:42

answered Aug 7 at 22:35

Marek R

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1 Comment

Otto Lewis Aug 8 at 10:41

I have taken this approach to use brace initialization, I also found std::underlying_type to be useful in some circumstances stackoverflow.com/a/14589519/10633627 for others who might be interested, thanks

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