CODEWITHSUNDEEP
c++cvisual-studio
happysaint
happysaint

Reputation: 71

Unexpected output in converting from long in C to long in C++ in VS?

I'm writing a program in C (with a school-specific IDE), then converting it to C++ in Visual Studio. The program in C takes a long variable as user input of 371449635398431 (using the school IDE and a unique compiler function called get_long) When I copy this to Visual Studio, using cin >> instead of get_long, it interprets the long as 2147483647 unless I make it a long long in VS. Are longs in C different than longs in C++ (with longs in C being equivalent to long long in C++?), or could a compiler mismatch be causing the issue?

Upvotes: 2

Views: 88

Answers (3)

supercat
supercat

Reputation: 81179

When the first C Standard was written, it was common for compilers to interpret "long" as the shortest type with at least 32 bits. When compilers added support for 64-bit types, it was common for "long long" to be the shortest type with at least 64 bits. Treating things in this way meant that the lack of fixed-length types wasn't really a problem, since code needing a 32-bit type could use "long" on any platform that had a 32-bit integer type.

MSVC has always followed that convention, even when targeting 64-bit platforms. Some other compilers, however, have decided to change the meaning that long had always had on microcomputer-based platforms, so that instead of long being 32 bits and long long being 64 bits, both would be 64-bit types.

Note, btw, that even on platforms where long and long long are both 64-bit types, they're not compatible with each other. If one has some functions that operate on data identified via long*, and others that operate on data identified via long long*, both clang and gcc will assume that those pointers will never be used to access the same storage.

Upvotes: 3

Evgeny
Evgeny

Reputation: 1072

The size of long depends of compiler, settings, data model and phase of moon (the last reason is a joke). But the long size is at least 32 bit.

If you want to use exact size you can use int64_t. for example.

Upvotes: 2

Bathsheba
Bathsheba

Reputation: 234715

The C and C++ standards say that a long must have a range of at least -2147483647 to +2147483647. (Note that C++20 requires -2147483648 as the minimum).

Microsoft's compiler for Windows has the range for a long set to -2147483648 to +2147483647. A typical Unix compiler (gcc on rhel for example) has a 64 bit type for a long.

The motivation for keeping the long as a 32 bit type on Windows 64 bit was for backward compatibility. It was really easy to convert Win32 applications to Win64 applications, back in the day.

Upvotes: 6

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