{"id":62,"date":"2004-09-23T00:59:04","date_gmt":"2004-09-23T08:59:04","guid":{"rendered":"\/?p=62"},"modified":"2010-01-18T02:42:16","modified_gmt":"2010-01-18T10:42:16","slug":"pitfalls-of-cpp-subscript-operator","status":"publish","type":"post","link":"https:\/\/www.slimjimmy.com\/weblog\/archives\/2004\/09\/23\/pitfalls-of-cpp-subscript-operator\/","title":{"rendered":"Pitfalls of C++’s subscript operator"},"content":{"rendered":"

What’s wrong with this C++ code?<\/p>\n

\r\n#include <iostream>\r\n#include <string>\r\n
\r\nclass Foo\r\n{\r\npublic:\r\n    Foo()\r\n    {\r\n    }\r\n
\r\n    operator bool()\r\n    {\r\n        return true;\r\n    }\r\n
\r\n    std::string operator[](const std::string& s)\r\n    {\r\n        return s;\r\n    }\r\n};\r\n
\r\nint main(void)\r\n{\r\n    Foo foo;\r\n    std::cout << foo[\"hello world!\"] << std::endl;\r\n    return 0;\r\n}\r\n<\/pre>\n
Answer:<\/p>\n
<\/p>\n
Trying to compile the above code with gcc produces the following error:<\/p>\n
\r\nIn function `int main()':\r\nambiguous overload for `Foo &[const char[13]]'\r\ncandidates are: operator [](int, const char *) <builtin>\r\n                class string Foo::operator [](const string &)\r\n<\/pre>\n
Wha—?  Where the heck did operator[](int, const char*)<\/code> come from?<\/p>\n
This little bit of goofiness is inherited from C.  In C, the subscript operator is unintuitively commutative<\/a>.  That is, ptr[index]<\/code> is exactly the same as index[ptr]<\/code>!<\/p>\n
In C, this is never a problem.  Nobody writes code like index[ptr]<\/code>, and you can be blissfully ignorant that it’s allowed at all.<\/p>\n
In C++, it can manifest itself where you don’t expect it, and this is what happens in the code above.<\/p>\n
The compiler tries to parse the expression foo[\"hello world!\"]<\/code>.  Foo<\/code> doesn’t have an operator[](const char*)<\/code> method (nor an operator[](const char[13])<\/code> one), so the compiler has two choices:<\/p>\n