C++ 与字符串文字比较会导致未指定的行为？
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Comparison with string literal results in unspecified behaviour?
提问by user1742497
I am having a problem with the program I am trying to code. It's just a Windows console program and I am very new to C++. It's only my 4th program.
我正在尝试编写的程序有问题。它只是一个 Windows 控制台程序，我对 C++ 很陌生。这只是我的第四个程序。
The problem I am having is that when I run my program I have no errors but a lot of warnings that say "comparison with string literal results in unspecified behaviour" in the lines that I will highlight below.
我遇到的问题是，当我运行我的程序时，我没有错误，但是在我将在下面突出显示的行中，有很多警告说“与字符串文字比较导致未指定的行为”。
When the program runs instead of adding the numbers I want it to it just gives me a random huge number no matter what I put in for my inputs.
当程序运行而不是添加我想要的数字时，它只会给我一个随机的巨大数字，无论我输入什么。
Here is the code:
这是代码：
#include <iostream>
using namespace std;
int main()
{
int hold;
int i;
int n;
i = 6;
int result;
int * price;
char items[100][100];
if (items == 0)
cout << "No items can be stored";
else
{
for (n=0; n<i; n++)
{
cout << "Item#" << n << ": ";
cin >> items[n];
}
cout << "\nYou Entered: \n";
for (n=0; n<i; n++)
cout << items[n] << ", ";
}
for (n=0; n<i; n++)
{
if (items[n] == "ab"){
price[n] = 2650;
}
else if (items[n] == "ae"){
price[n] = 1925;
}
else if (items[n] == "ie"){
price[n] = 3850;
}
else if (items[n] == "bt"){
price[n] = 3000;
}
else if (items[n] == "pd"){
price[n] = 2850;
}
else if (items[n] == "ga"){
price[n] = 2600;
}
}
for (n=0; n<i; n++)
{
result = result + price[n];
}
cout << "\nTotal gold for this build: " << result;
cin >> hold;
return 0;
}
Any help is appreciated. There is probably something big that I've done wrong. The names in the if statements are all currently placeholders and I'll be adding a lot more if statements when I can get it to work with the bare 6 which is what it needs to work.
任何帮助表示赞赏。我可能做错了什么大事。if 语句中的名称目前都是占位符，当我可以让它与裸 6 一起工作时，我将添加更多 if 语句，这是它需要的工作。
回答by BigBoss
In C++ ==only implemented internally for primitive types and array is not a primitive type, so comparing char[100]and string literal will only compare them as 2 char*or better to say as 2 pointers and since this 2 pointers can't be equal then items[n] == "ae"can never be true, instead of this you should either use std::stringto hold string as:
在 C++ 中==仅在内部为原始类型实现，数组不是原始类型，因此比较char[100]和字符串文字只会将它们比较为 2char*或更好地说为 2 个指针，并且由于这 2 个指针不能相等，items[n] == "ae"因此永远不会为真，而不是这个，您应该使用std::string将字符串保存为：
std::string items[100];
// initialize items
if( items[n] == "ae" ) ...
or you should use strcmpto compare strings, but remeber strcmpreturn 0 for equal strings, so your code will be as:
或者您应该使用strcmp比较字符串，但请记住strcmp对于相等的字符串返回 0，因此您的代码将如下所示：
char items[100][100];
// initialize items
if( strcmp(items[n], "ae") == 0 ) ...
And one extra note is if (items == 0)is useless, since itemsallocated on stack and not in the heap!
额外的注释是if (items == 0)无用的，因为items分配在堆栈中而不是在堆中！
回答by Luchian Grigore
First, int * price;is a dangling pointer - you never initialize it. You have to do:
首先，int * price;是一个悬空指针——你永远不会初始化它。你必须要做：
int * price = new int[i];
Second, usually, idenotes an iterator index so I suggest you stick with that - so
其次，通常，i表示迭代器索引，所以我建议你坚持 - 所以
for (i=0; i<n; i++) //some more refactoring needed
Third, you need to compare char arrays using strncmpin your case.
第三，您需要比较strncmp在您的情况下使用的 char 数组。
Fourth and most important- use std::stringand std::vectorinstead. This is C++, not C.
第四也是最重要的- 使用std::stringandstd::vector代替。这是 C++，不是 C。
回答by m0skit0
You're comparing pointers, not the actual strings. Use C++ stringclass instead of char*(or check how C strings work).
您正在比较指针，而不是实际的字符串。使用 C++string类代替char*（或检查C 字符串的工作方式）。
回答by alex
Just a little thing that got me stumbling for a bit, is the difference between single and double quotes, see: Single quotes vs. double quotes in C or C++
只是让我绊了一下的一件小事，是单引号和双引号之间的区别，请参阅：单引号与 C 或 C++ 中的双引号
I was comparing the first character of a string with double quotes and not single quotes - which resulted in above's error message.
我正在将字符串的第一个字符与双引号而不是单引号进行比较 - 这导致了上面的错误消息。
