# 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++編譯上面的例子，會報如下錯誤： $ 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] 這是一個強制類型轉換的錯誤，你可以修改源代碼為： std::cout << (int*)(this) << "\n"; 即可。 如果，你不想（或不能）去修改源程序，只是應為升級了gcc而帶來了這樣的錯誤，那么也可以使用`-fpermissive`選項，將錯誤降低為警告： $ 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] 詳情參見[gcc手冊](https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-fpermissive-166) ## 貢獻者 xmj