error cast from ... to ... loses precision

例子

#include <iostream>

class Foo {
 public:
  void print() const {
    std::cout << (int)(this) << "\n";
  }
};

int main()
{
  class Foo foo;

  foo.print();
  return 0;
}

技巧

在g++编译上面的例子，会报如下错误：

$ g++ foo.cc 
foo.cc: In member function ‘void Foo::print() const’:
foo.cc:6:28: error: cast from ‘const Foo*’ to ‘int’ loses precision [-fpermissive]

这是一个强制类型转换的错误，你可以修改源代码为：

std::cout << (int*)(this) << "\n";

即可。

如果，你不想（或不能）去修改源程序，只是应为升级了gcc而带来了这样的错误，那么也可以使用-fpermissive选项，将错误降低为警告：

$ g++ foo.cc -fpermissive
foo.cc: In member function ‘void Foo::print() const’:
foo.cc:6:28: warning: cast from ‘const Foo*’ to ‘int’ loses precision [-fpermissive]

详情参见 gcc手册