连接两个字符串文字 [英] Concatenate two string literals

问题描述

我是编程的新手，我正在读Koenig的Accelerated C ++。无论如何，我正在学习字符串，他写道新的想法是，我们可以使用+来连接一个字符串和一个字符串字面量 - 或者说，两个字符串（但不是两个字符串字面量）。

I am very new to programming, and am reading Accelerated C++ by Koenig. Anyways, I am learning about strings and 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).

好，这是有道理的，现在分两个单独的练习意在照亮这个。

Fine, this makes sense I suppose. Now onto two separate exercises meant to illuminate this .

以下定义是否有效？

const string hello = "Hello";

const string message = hello + ",world" + "!";

现在，我尝试执行上述操作， 。

Now, I tried to execute the above and it worked! So I was happy.

然后我试着做下一个练习：

Then I tried to do the next exercise;

const string exclam = "!";

const string message = "Hello" + ",world" + exclam;

这没有效果，现在我明白，它与你不能连接两个字符串文字的事实有关，但我不明白为什么我设法得到第一个示例工作（不是，世界和！两个字符串文字？

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.

谢谢！

推荐答案

const string message = "Hello" + ",world" + exclam;

+ 右结合性，所以等效的括号表达式是：

The + operator has left-to-right associativity, so the equivalent parenthesized expression is:

const string message = (("Hello" + ",world") + exclam);

正如你所看到的，两个字符串文字Hello和，世界首先是添加，因此出错。

As you can see, the two string literals "Hello" and ",world" are "added" first, hence the error.

前两个串联的字符串必须是 std :: string 对象：

One of the first two strings being concatenated must be a std::string object:

const string message = string("Hello") + ",world" + exclam;

或者，您可以强制第二个 +

Alternatively, you can force the second + to be evaluated first by parenthesizing that part of the expression:

const string message = "Hello" + (",world" + exclam);

你的第一个例子（ hello +，world ！），因为 std :: string （ hello ）最左边的参数 + 。 + 被计算，结果是一个 std :: string 对象与连接的字符串， c $ c> std :: string 然后与！连接。

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 "!".

至于为什么，您不能使用 + 连接两个字符串文字，一个字符串字面量只是一个字符数组（ const char [N] 其中 N 加一，为null终止符）。当您在大多数上下文中使用数组时，它将转换为指向其初始元素的指针。

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.

因此，当您尝试执行Hello+，world做的是添加两个 const char * s在一起，这是不可能的（什么意思是添加两个指针在一起？），如果它是不会做你想要什么。

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.

请注意，您可以通过将字符串文字放在彼此旁边来连接;例如，以下两个是等效的：

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"

如果你有一个长字符串要分解成多行，这很有用。它们必须是字符串字面量，但是：这不会与 const char * 指针或 const char [N] 数组。

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.

这篇关于连接两个字符串文字的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！