C++ STL用string类的时候 编译器提示 error: no viable overloaded ‘=’，等于号不能重载//周末自己研究研究

C++ STL用string类的时候 编译器提示 error: no viable overloaded ‘=’，等于号不能重载

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...   if(s1_len > s2_len)     for(int i = 1; i <= s1_len - s2_len; i++)       s2 = '0' + s2;

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main.cpp:41:10: error: no viable overloaded '='       s2 = s2 + "0";       ~~ ^ ~~~~~~~~ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1330:19: note:       candidate function not viable: 'this' argument has type 'const string' (aka 'const basic_string<char,       char_traits<char>, allocator<char> >'), but method is not marked const     basic_string& operator=(const basic_string& __str);                   ^ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1337:45: note:     _LIBCPP_INLINE_VISIBILITY basic_string& operator=(const value_type* __s) {return assign(__s);}                                             ^ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/string:1338:19: note:     basic_string& operator=(value_type __c);