C ++三元运算符字符串连接

Question

I have seen this question asked for other languages, but not C++. Here's what I'm trying to do:

bool a = true;
string s;
s.append("a:" + (a? "true" : "false"));    
cout << s << endl;

I get a compiler error stating that I cannot add two pointers. What gives?

Answer 1

s.append(string("a:") + (a? "true" : "false"));

Answer 2

"true" , "false" , "a:" , "b:" and "c:" are pointers (raw char arrays, actually). When you add them together with + (eg "a:" + "true" ) , then std::string is not involved, and it's only std::string which actually gives + the meaning of concatenation.

Here's what I do in such situations:

s += "a:" + std::string(a ? "true" : "false");

Answer 3

Just noting this, but when C++14 comes around, this will become even easier. Firstly, this can currently be written using operator+= instead of append :

s += std::string("a:") + (a ? "true" : "false"); 

In C++14, there will be a standard operator""s , which means this is possible:

s += "a:"s + (a ? "true" : "false"); 

However, the one that still makes the most sense to use in both cases, based on the shown code, is DigitalEye's answer , since it removes the need for a cast altogether. I imagine your real code is different, though, which means this could soon be useful (or already, given a C++14 standard library implementation being in use).

Answer 4

Try

 bool a;

 string s;
 s.append(string("a:") + (a? "true" : "false"));

Answer 5

Because you try to concatenate two strings which are arrays of type char . "a:" is of type const char[3] and you cannot concatenate such strings with + operator. You have to use type string which has defined + operator which can be used to concatenate strings:

s.append(std::string("a:") + (a? "true" : "false"));

Answer 6

"a:" is char const[3] and (a? "true" : "false") is also char const[5] (for true) or char const[6] (for false), so you are applying + operator to two pointers.

The code below will work, because you are calling the overloaded += of string class that takes char const* as parameter:

s = "a:"
s +=  a ? "true": "false";