在C++中，有些情況下表達式的值被自動轉換為true或false，比如int，double，char等類型，本文給出一些測試，比如0.0和'\0'會被自動轉換成false。
1 #include <iostream>
2
3 #define T "true"
4 #define F "false"
5
6 using namespace std;
7
8 int main() {
9 int intt = -2;
10 int intf = 0;
11 double dt = 0.1;
12 double df = .0;
13 char ct = 'a';
14 char cf = '0';
15 char cf0 = '\0';
16 char cfn = '\n';
17 char cfr = '\r';
18 char cfb = '\b';
19 string st = "2";
20 string sf = "0";
21 bool bt = true;
22 bool bf = false;
23
24 cout << "-2 == " << (intt ? T : F) << '\n';
25 cout << "0 == " << (intf ? T : F) << '\n';
26 cout << "0.1 == " << (dt ? T : F) << '\n';
27 cout << "0.0 == " << (df ? T : F) << '\n';
28 cout << "\'a\' == " << (ct ? T : F) << '\n';
29 cout << "\'0\' == " << (cf ? T : F) << '\n';
30 cout << "\'\\0\' == " << (cf0 ? T : F) << '\n';
31 cout << "\'\\n\' == " << (cfn ? T : F) << '\n';
32 cout << "\'\\r\' == " << (cfr ? T : F) << '\n';
33 cout << "\'\\b\' == " << (cfb ? T : F) << '\n';
34 cout << "\"2\" == " << (st ? T : F) << '\n'; //error: could not convert ‘st’ to ‘bool’
35 cout << "\"0\" == " << (sf ? T : F) << '\n'; //error: could not convert ‘sf’ to ‘bool’
36 cout << "true == " << (bt ? T : F) << '\n';
37 cout << "false == " << (bf ? T : F) << '\n';
38
39 return 0;
40 }
41
g++ 4.4.5編譯，報錯，因為在第34和35行中，string無法被自動轉換成bool類型。把第34和35行注釋掉，再編譯，通過。以下是結果：
-2 == true
0 == false
0.1 == true
0.0 == false
'a' == true
'0' == true
'\0' == false
'\n' == true
'\r' == true
'\b' == true
true == true
false == false
可以看出，double的0.0和字符型中的'\0'被自動轉換成了false，注意，'0'被自動轉換成了true。