Non standard return fails to compile

[zofer1]

This statement:
return BOOST_ASSERT_MSG( i < N, "out of range" ), elems[i];

is a non-standard form of return.

In case we compile optimized code that lacks assert function but still want something this code fails to compile.

Separating this to a more standard form solves the issue:

BOOST_ASSERT_MSG( i < N, "out of range" );
return elems[i];

[pdimov]

There's nothing non-standard about it.

What's the actual error you're getting, and under what circumstances does it occur?

[zofer1]

Hi, The error we get is this: For below failure 11:33:01 [INFO] [echo] .sb/tp_include/boost/array.hpp:109:20: error: expected primary-expression before 'if' 11:33:01 [INFO] [echo] return BOOST_ASSERT_MSG( i < N, "out of range" ), elems[i]; 11:33:01 [INFO] [echo] While this form of return with two arguments adheres to the C++ standard, the common use of return statement is with a single argument. I could not find a simple example here: https://en.cppreference.com/w/cpp/language/return.html It took me some time to understand how it works. If I understand the context here, the first expression that evaluates to true is used for return. As we wish to have here an assert with this message when using debug versions. Anyways, unless you are truly familiar C++ and try to make some sense of it, you will find it confusing. If we really want to have an assert message and avoid accessing array items beyond the array boundaries and potential segmen tviolation, there are more readable ways to do that. If this is really interesting I will look up the exact flags combination that puts us in this compilation failure, but this should be modified for clarity regardless. Cheers, Zohar

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On Tue, 28 Oct 2025 at 17:46, Peter Dimov ***@***.***> wrote: *pdimov* left a comment (boostorg/array#29) <#29 (comment)> There's nothing non-standard about it. What's the actual error you're getting, and under what circumstances does it occur? — Reply to this email directly, view it on GitHub <#29 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABU4EA6FLP2ICJ2HSSLAEBL3Z6FXFAVCNFSM6AAAAACKOMCLTGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTINJXGE2DKNZXG4> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[pdimov]

11:33:01 [INFO] [echo] .sb/tp_include/boost/array.hpp:109:20: error:
expected primary-expression before 'if'

This means that you're defining BOOST_ASSERT_MSG to something with if in it. This is not the definition in boost/assert.hpp; you must be getting it from somewhere else.

While this form of return with two arguments adheres to the C++ standard,
the common use of return statement is with a single argument.

This is not a two argument return; it's a comma expression.

https://en.cppreference.com/w/cpp/language/operator_other.html

(See section "Built-in comma operator".)

[mclow]

Some versions of C++ require that constexpr functions consist of only a single statement.

Rewriting the code as you suggest would mean that it was two statements, and could no longer be used in constant expressions.