Is sizeof(bool) defined in the C++ language standard?

Question

I can't find an answer in the standard documentation. Does the C++ language standard require sizeof(bool) to always be 1 (for 1 byte), or is this size implementation-defined?

Cube

If your code depends on bool to be 1 byte, you can check it at compile-time: static_assert(sizeof(bool) == 1, "OMG bool is big here");. – Terrill

Answer 1

sizeof(bool) is implementation defined, and the standard puts notable emphasis on this fact.

§5.3.3/1, abridged:

sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1; the result of sizeof applied to any other fundamental type is implementation-defined. [Note: in particular, sizeof(bool) and sizeof(wchar_t) are implementation-defined.⁶⁹⁾]

Footnote 69):

sizeof(bool) is not required to be 1.

Crassus

is there a flag that i need to compile my program with, that my compiler will use only 1 byte for bool? – Petronille

@Eagle: That's up to your compiler, I'm not sure. It's probably best you left it up to your compiler. – Crassus

note that std::vector<bool> is optimized to a vector containing 1bit bools by the standard. – Tippett

@Tippett Any reference? – Landing

@Tippett is it standard really? This mentions implementation-dependence – Rainfall

Answer 2

http://msdn.microsoft.com/en-us/library/tf4dy80a.aspx

"In Visual C++4.2, the Standard C++ header files contained a typedef that equated bool with int. In Visual C++ 5.0 and later, bool is implemented as a built-in type with a size of 1 byte. That means that for Visual C++ 4.2, a call of sizeof(bool) yields 4, while in Visual C++ 5.0 and later, the same call yields 1. This can cause memory corruption problems if you have defined structure members of type bool in Visual C++ 4.2 and are mixing object files (OBJ) and/or DLLs built with the 4.2 and 5.0 or later compilers."

Fredericafrederich

You do realize that this is somewhat related to the question, but clearly not an answer, since 0xbadf00d asked about the standard, not some specific/arbitrarily selected compiler implementation, right? – Guillotine

@ChristopherCreutzig It is a proof by counterexample. – Selaginella

@Selaginella It's not a counterexample. There are many things in Visual C++ that are not standard-compliant. – Cube

Answer 3

It's implementation defined. Only sizeof(char) is 1 by the standard.

Tonie

pls note that the 1 in the standard can mean 4 byte. Than every type is a product of 4. So care that the standard ONLY defines char is the 1, but not defines the measurment. – Tippett

1 means 8 bits or 1 byte in the standard – Predacious

1 byte. The number of bits per byte is not defined by the standard (it needs to be at least 8 IIRC), but can be found in CHAR_BIT, defined in climits. – Tonie

sizeof(unsigned char) and sizeof(signed char) also have to be 1 by the standard – Augend

Answer 4

See 5.3.3 paragraph 1 :

[Note: in particular, sizeof(bool) and sizeof(wchar_t) are implementation-defined.69) ]

Cutright