使用静态常量初始化 unique_ptr 时出现未定义引用错误

Question

当我尝试使用 static const 初始化 unique_ptr 时，出现 "undefined reference" 错误。然而，当我new一个指针使用相同的常量时，这个符号似乎被神奇地定义了。

这是一个重现错误的简单程序：

Outside_library.h

class Outside_library
{
public:
  static const int my_const = 100;
};

main.cpp

#include "Outside_library.h"

#include <iostream>
#include <memory>

class My_class
{
public:
  My_class(int num)
  {
    m_num = num;
  };

  virtual ~My_class(){};

private:
  int m_num;
};

int main(int, char* [])
{
  My_class* p_class = new My_class(Outside_library::my_const);
  delete p_class;

  // ==== ERROR HERE: When I comment this line out, the program runs fine.
  std::unique_ptr<My_class> p_unique
      = std::make_unique<My_class>(Outside_library::my_const);

  std::cout << "I made it through!" << std::endl;
  return 0;
}

我使用

编译程序

g++ main.cpp -std=c++14

并出现以下错误。

/tmp/ccpJSQJS.o: In function `main':
main.cpp:(.text+0x51): undefined reference to `Outside_library::my_const'
collect2: error: ld returned 1 exit status

谁能帮我理解为什么在使用 new 时定义了常量，而在使用 make_unique 时却没有？

Answer 1

make_unique 获取其参数的 (forwardind) 引用，因此 odr-使用它。

My_class(Outside_library::my_const)只使用值。

一种解决方案是在 TU 中定义成员：

const int Outside_library::my_const;

~~或使用 constexpr 值 (C++11 起):~~

~~class Outside_library { public: static constexpr int my_const = 100; };~~

Answer 2

C++ 有一个叫做 One-Definition Rule (ODR):

的东西

Informally, an object is odr-used if its value is read (unless it is a compile time constant) or written, its address is taken, or a reference is bound to it; a reference is odr-used if it is used and its referent is not known at compile time; and a function is odr-used if a function call to it is made or its address is taken. If an object, a reference or a function is odr-used, its definition must exist somewhere in the program; a violation of that is usually a link-time error.

linked 站点给出了以下示例：

struct S {
    static const int x = 0; // static data member
    // a definition outside of class is required if it is odr-used
};
const int& f(const int& r);

int n = b ? (1, S::x) // S::x is not odr-used here
          : f(S::x);  // S::x is odr-used here: a definition is required

您的显式构造函数调用不会 "odr-use" Outside_library::my_const 但对 std::make_unique() 的调用会。当一个对象被 odr-used 时，它必须只有一个 definition（而不是 declaration）。您的示例只有一个声明。再次来自 cppreference:

a variable x in a potentially-evaluated expression ex is odr-used unless both of the following are true:

applying lvalue-to-rvalue conversion to x yields a constant expression that doesn't invoke non-trivial functions

either x is not an object (that is, x is a reference) or, if x is an object, it is one of the potential results of a larger expression e, where that larger expression is either a discarded-value expression or has the lvalue-to-rvalue conversion applied to it

Jarod42 建议的解决方案是使用 constexpr 而不是 const（如果您可以控制 "outside library" 代码）。如果不这样做，那么您将需要 link 针对包含 Outside_library::my_const.

的 definition 的库的程序

g++ main.cpp -std=c++14 -lOutside_library

使用静态常量初始化 unique_ptr 时出现未定义引用错误

Undefined reference error when initializing unique_ptr with a static const

c++

smart-pointers

c++14