Concatenate two string literals

Question

I am reading Accelerated C   by Koenig  He writes that  the new idea is that we can use   to concatenate a string and a string literal - or  for that matter  two strings  but not two string literals    Fine  this makes sense I suppose  Now onto two separate exercises meant to illuminate this     Are the following definitions valid   const string hello    Hello    const string message   hello     world           Now  I tried to execute the above and it worked  So I was happy   Then I tried to do the next exercise    const string exclam         const string message    Hello      world    exclam    This did not work  Now I understand it has something to do with the fact that you cannot concatenate two string literals  but I don t understand the semantic difference between why I managed to get the first example to work  isn t   world  and     two string literals  Shouldn t this not have worked   but not the second

User · Answer

In case 1  because of order of operations you get    hello      world         which resolves to hello       and finally to hello  In case 2  as James noted  you get     Hello       world     exclam which is the concat of 2 string literals   Hope it s clear

User · Answer

Your second example does not work because there is no operator   for two string literals  Note that a string literal is not of type string  but instead is of type const char    Your second example will work if you revise it like this   const string message   string  Hello       world    exclam

User · Answer

Since C  14 you can use two real string literals   const string hello    Hello s   const string message   hello     world s      s    or  const string exclam      s   const string message    Hello s     world s   exclam

User · Answer

const string message    Hello      world    exclam    The   operator has left-to-right associativity  so the equivalent parenthesized expression is   const string message      Hello      world     exclam     As you can see  the two string literals  Hello  and   world  are  added  first  hence the error   One of the first two strings being concatenated must be a std  string object   const string message   string  Hello       world    exclam    Alternatively  you can force the second   to be evaluated first by parenthesizing that part of the expression   const string message    Hello       world    exclam     It makes sense that your first example  hello     world         works because the std  string  hello  is one of the arguments to the leftmost     That   is evaluated  the result is a std  string object with the concatenated string  and that resulting std  string is then concatenated with the         As for why you can t concatenate two string literals using    it is because a string literal is just an array of characters  a const char  N  where N is the length of the string plus one  for the null terminator    When you use an array in most contexts  it is converted into a pointer to its initial element     So  when you try to do  Hello      world   what you re really trying to do is add two const char s together  which isn t possible  what would it mean to add two pointers together   and if it was it wouldn t do what you wanted it to do      Note that you can concatenate string literals by placing them next to each other  for example  the following two are equivalent    Hello    world   Hello world    This is useful if you have a long string literal that you want to break up onto multiple lines   They have to be string literals  though   this won t work with const char  pointers or const char N  arrays

User · Answer

The difference between a string  or to be precise  std  string  and a character literal is that for the latter there is no   operator defined  This is why the second example fails   In the first case  the compiler can find a suitable operator  with the first argument being a string and the second a character literal  const char   so it used that  The result of that operation is again a string  so it repeats the same trick when adding     to it

User · Answer

You should always pay attention to types   Although they all seem like strings   Hello  and   world  are literals   And in your example  exclam is a std  string object   C   has an operator overload that takes a std  string object and adds another string to it  When you concatenate a std  string object with a literal it will make the appropriate casting for the literal   But if you try to concatenate two literals  the compiler won t be able to find an operator that takes two literals

[c++] Concatenate two string literals

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