为什么不为C ++ 17放弃一个定义规则？

Question

Citing C++ Draft N4713:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (9.4.1); no diagnostic required. The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined (see 15.1, 15.4 and 15.8). An inline function or variable shall be defined in every translation unit in which it is odr-used outside of a discarded statement.

In C++ versions prior to C++17, I can get around this restriction by just declaring my functions inline . C++17 adds the same feature for variables.

Furthermore, it seems to me that the inline -Keyword does not serve another purpose apart from making it possible to ignore the ODR.

So, why isn't this whole rule just abandoned for C++17 ? I can't see the purpose of a rule that can be turned off.

Answer 1

"Turning off" the ODR with inline is not free: the definition of an inline entity must be present in every translation unit. Note that this implies that any change to its definition causes re-compilation of every compilation unit that uses it. This would be particularly unpleasant when the function is part of some library many / big projects depend upon.

Non- inline functions, on the other hand, live in just one compilation unit and are referenced via some symbol by the linker when needed elsewhere. Complying with the ODR guarantees that symbol is not ambiguous.

Answer 2

inline is dangerous and expensive.

It is expensive because every compilation unit that uses something now depends on the definition of the thing. So change the body? Recompile every user of it.

It is dangerous because if two inline definitions disagree, your program is IF-NDR (ill formed, no diagnostic required).

Without inline , two definitions cause an ill formed program, but the compiler must provide a diagnostic; usually a hard error.

inline turns off that extremely useful warning.

If every compiler was capable of converting the IF-NDR of different inline definitions into a diagnostic error message you'd have more of a case. So long as that proves difficult and/or not implemented, inline is a "activate unsafe mode!" option. Making it default would be counter productive.

Answer 3

The trade-off is that you need the definition of an inline function everywhere it's used. If you want that, just put your whole program in a single .cpp file.

The ODR is what you need to have separate compilation, and that is still useful.

Answer 4

From cppreference :

One and only one definition of every non-inline function or variable that is odr-used (see below) is required to appear in the entire program (including any standard and user-defined libraries). The compiler is not required to diagnose this violation, but the behavior of the program that violates it is undefined.

Declaring a function as inline does not "ignore" ODR, but it causes each appearance of the function to be its own entity that needs a definition in each translation unit where it is used. Small, but significant difference. ODR is still required to have seperate translation units.