ODR和内部链接

Question

假设我在一个程序中有两个编译单元，每个编译单元都声明了一个非内联函数，具有相同的签名，但实现不同，例如

// a.cpp
namespace internal {
    int foo(int a) {
        return a+1;
    }
}

int main() {
}

和

// b.cpp
namespace internal {
    int foo(int b) {
        return b+2;
    }
}

Compiling/linking 这个（g++ 4.8.3 with -std=c++11），我得到一个错误

b.cpp:(.text+0x0): multiple definition of `internal::foo(int)'

这是意料之中的，因为据我所知，这只是违反了 one definition rule:

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).

现在，将 namespace internal 更改为未命名的命名空间，错误消失了。直觉上，这对我来说很有意义，因为我正在改变函数 external to internal linkage:

Any of the following names declared at namespace scope have external linkage unless the namespace is unnamed or is contained within an unnamed namespace (since C++11): variables and functions not listed above (that is, functions not declared static [...]) ...

和

[A]ll names declared in unnamed namespace or a namespace within an unnamed namespace, even ones explicitly declared extern, have internal linkage.

但是，我无法在一个定义规则中找到任何免除具有内部链接的函数的内容。因此，我的问题是：我的直觉推理是否正确，或者我是否仍然违反具有内部链接的函数的单一定义规则（并且 compiler/linker 只是不再报告）？另外，标准（或cppreference.com :)）在哪里说是否可以？

Answer 1

n4713

§6.5 Program and linkage [basic.link]

A name is said to have linkage when it might denote the same object, reference, function, type, template, namespace or value as a name introduced by a declaration in another scope:

(2.1) — When a name has external linkage, the entity it denotes can be referred to by names from scopes of other translation units or from other scopes of the same translation unit.

(2.2) — When a name has internal linkage, the entity it denotes can be referred to by names from other scopes in the same translation unit.

— When a name has no linkage, the entity it denotes cannot be referred to by names from other scopes.

这基本上是说（在未命名命名空间的情况下）来自 a.cpp 的名称 foo 和来自 b.cpp 的名称 foo 各自指代不同的实体。所以你没有同一个对象的两个定义，所以没有违反 ODR。

ODR和内部链接

ODR and internal linkage

c++

c++11

linkage

one-definition-rule