CODEWITHSUNDEEP
c++ubuntucompilationoperating-system
user10333388
user10333388

Reputation: 23

C++ code compiles differently on different OS

I was wondering so why does c++ code compile differently on different version of the OS. Such as when the same code is complied on the OS no warning or anything will be brought up, but when the same code is complied on a different OS, there will be warnings or errors.

So why does this happen. Is the difference between gcc versions or what actually makes the c++ code unique when its complied on two different OS such Ubuntu 14 and Ubuntu 16. I am just trying to understand how the c++ code is unique to the OS compilation.

Upvotes: 2

Views: 519

Answers (1)

alter_igel
alter_igel

Reputation: 7202

C++ as a language is defined by its standard. The standard is an enormous, lawyer-lingo document that defines the language's syntax, rules, standard library, and some guidelines for how compilers should correctly process source code. Compilers, the bridge between the abstract language and real, executable programs, are implemented by different vendors or organizations, and should adhere to that standard as closely as possible. In practice, their correctness varies[1].

Many compiler errors are part of the standard (diagnostics in standardese), and so should in principle be essentially the same across compilers[2]. Compiler warnings generally are less technical, and are often ways that compiler vendors try to help you catch common programming errors that aren't technically ill-formed programs. A program may be ill-formed according to the standard, meaning that it is syntactically invalid and does not represent a real program. Compilers are required by the standard to issue a diagnostic for an ill-formed program.

There are however lesser, more subtle ways that programs can be incorrect though, for example by using what the standard refers to as undefined behavior (UB) and implementation-defined behavior. These are situations where the standard doesn't specify how a compiler should correctly translate source code into a program, and compiler vendors are legally allowed to proceed how they please. While many compilers are likely to produce code that does approximately what you expect it to, invoking undefined behavior in a program is generally a very bad idea because there's no guarantee of any kind how your program will behave. Code with UB that compiles quietly and passes tests on one compiler may fail tests or fail to compile altogether, or encounter a bug at the worst possible time, on a different compiler. The situation gets hairy too if you're using compiler-specific language extensions.

When faced with potential UB, some compilers may offer very helpful advice and others may be misleadingly silent. The best practice would be to be familiar with causes of UB by learning C++ from a good source and reading documentation carefully, both C++ language documentation and that of any libraries you may be using.

[1] Take a look at the 'Standard conformance' columns of the list of C++ compilers at https://en.wikipedia.org/wiki/List_of_compilers#C++_compilers

[2] A comparison of error messages and warnings from three very popular compilers: https://easyaspi314.github.io/gcc-vs-clang.html

Upvotes: 1

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