What is the size of float and double in C and C++? [duplicate]

Excerpt from the C99 standard, normative annex F (The C++-standard does not explicitly mention this annex, though it includes all affected functions without change per reference. Also, the types have to match for compatibility.):

IEC 60559 floating-point arithmetic

F.1 Introduction

1 This annex specifies C language support for the IEC 60559 floating-point standard. The
IEC 60559 floating-point standard is specifically Binary floating-point arithmetic for
microprocessor systems, second edition (IEC 60559:1989), previously designated
IEC 559:1989 and as IEEE Standard for Binary Floating-Point Arithmetic
(ANSI/IEEE 754−1985). IEEE Standard for Radix-Independent Floating-Point
Arithmetic (ANSI/IEEE 854−1987) generalizes the binary standard to remove
dependencies on radix and word length. IEC 60559 generally refers to the floating-point
standard, as in IEC 60559 operation, IEC 60559 format, etc. An implementation that
defines __STDC_IEC_559__ shall conform to the specifications in this annex.356)
Where a binding between the C language and IEC 60559 is indicated, the
IEC 60559-specified behavior is adopted by reference, unless stated otherwise. Since
negative and positive infinity are representable in IEC 60559 formats, all real numbers lie
within the range of representable values.

So, include <math.h> (or in C++ maybe <cmath>), and test for __STDC_IEC_559__.

If the macro is defined, not only are the types better specified (float being 32bits and double being 64bits among others), but also the behavior of builtin operators and standard-functions is more specified.
Lack of the macro does not give any guarantees.

For x86 and x86_64 (amd64), you can rely on the types float and double being IEC-60559-conformant, though functions using them and operations on them might not be.

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