cmath vs math.h (And similar c-prefixed vs .h extension headers)

The headers whose names start with c are derived from the headers of the C standard library. The corresponding headers with the c prefix removed and a .h suffix added are identical (or very nearly identical) to the C standard library headers.

<cmath> defines the relevant symbols under the std namespace; <math.h> defines them globally.

(I just learned it's not quite that simple; see Alf's answer.)

I've seen some information about differences between things like iostream vs iostream.h.

[iostream.h] is not a standard header.

it is not an example of the issue you're raising.

[cmath] defines symbols in the std namespace, and may also define symbols in the global namespace. [math.h] defines symbols in the global namespace, and may also define symbols in the std namespace. if you include the former and use an unqualified symbol, it may compile with one compiler but not with another. therefore it's a good idea to use [math.h]. and in general, for such header pairs, to use the [.h] version.

c++98 provided a formal guarantee of the cxxx header not polluting the global namespace. maybe that was why they were defined. however, that was a bit harder to implement than polluting ones, so in practice no standard library implementation that i know of followed the standard in this respect, and so it was finally changed to reflect reality in c++11.

Maybe this would be helpful :

The C++ library includes the same definitions as the C language library organized in the same structure of header files, with the following differences:

1 - Each header file has the same name as the C language version but with a "c" prefix and no extension. For example, the C++ equivalent for the C language header file < stdlib.h > is < cstdlib>.

2 - Every element of the library is defined within the std namespace.

c-prefixed vs .h extension headers