C++ interop type mapping for integer and floating-point literals

Table of contents

• Overview
• Carbon to C++ type mapping
  • Integer literals
  • Floating-point literals
• C++ to Carbon type mapping
  • Integer literals
  • Floating-point literals
• Alternatives considered
• References

Overview

The behavior of literals that participate in interop depends on the direction of interop, but generally follows C++ rules:

• Carbon literal to C++ type: A Carbon literal will be given a C++ type according to the C++ rules when used in C++ context, such as calling a C++ function.
• C++ literal to Carbon type: A C++ literal will be given a C++ type following the C++ rules, which will then be mapped to a Carbon type as defined in primitive types mapping.
  • For example, 1 becomes int according to C++ rules, and int maps to i32 in Carbon.

This is used:

• During overload resolution of C++ functions, when a C++ function is called from Carbon with a literal as an argument.
• When importing C++ macros to Carbon, such as macros defining constants where the constant is a literal.

Carbon to C++ type mapping

Integer literals

Because there are no suffixes in Carbon for the integer types, a Carbon decimal integer literal will follow the C++ rules for decimal integers. Carbon also doesn't make a distinction between hexadecimal and binary literals for interop, so these literals will also follow the C++ rules for decimal integers without a suffix.

The first type from the following list in which the literal's value can fit is selected:

• int
• long
• long long
• __int128

If the value doesn’t fit in any of these types, the program is ill-formed and diagnosed with an error. This matches the C++ standard behavior rather than Clang's -Wimplicitly-unsigned-literal behavior.

Because decimal numbers are most commonly used as integer literals, this should match most existing C++ calls. To match a C++ call with a non-decimal literal argument expecting an unsigned type, an explicit unsigned type must be provided in Carbon. For example:

Cpp.f(0xDEADBEEF as u32);

Floating-point literals

As there are no suffixes in Carbon for the floating-point literals, a Carbon floating-point literal will map to C++ double. If the value doesn’t fit in double, the program is ill-formed.

C++ to Carbon type mapping

Integer literals

A C++ integer literal will be given a C++ integer type following the C++ rules, which is then mapped to a Carbon type as defined in the primitive types mapping.

Floating-point literals

A C++ floating literal will be given a C++ type following the C++ rules, which is then mapped to a Carbon type as defined in primitive types mapping. :

Alternatives considered

Carbon to C++ type mapping

• Use the C++ standard rules for hexadecimal and binary literals
• Use the Clang way of determining the type

C++ to Carbon type mapping

• Use Carbon literal types

References

• Proposal #144: Numeric literal semantics
• Proposal #5448: Carbon / C++ interop primitive types
• Proposal #6668: C++ interop type mapping for integer and floating-point literals