C++ 错误——表达式必须具有整数或枚举类型——从带有连接的字符串中获取它？
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C++ error -- expression must have integral or enum type -- getting this from a string with concatenation?
提问by SuperCow
C++ error expression must have integral or enum typegetting this from a string with concatenation?
C++ 错误表达式必须具有整数或枚举类型，从带有连接的字符串中获取它？
So in the toString()of a class in C++ I have the code:
所以在toString()C++ 的一个类中，我有代码：
string bags = "Check in " + getBags() + " bags";
I thought I could declare a string like this? (I'm coming from a Java background and trying to learn C++). The bagsis underlined in Visual Studio though and the problem is:
我以为我可以声明这样的字符串？（我来自 Java 背景并试图学习 C++）。将bags在Visual Studio强调，虽然和问题是：
expression must have integral or enum type.
表达式必须具有整数或枚举类型。
getBags()just returns an int.
getBags()只返回一个int.
Another example where this happens is with:
发生这种情况的另一个例子是：
string totalPrice = "Grand Total: " + getTotalPrice();
getTotalPrice()returns a floatand is what is underlined with the error.
getTotalPrice()返回 afloat并且是错误下划线的内容。
But then if I put in a line like:
但是，如果我输入如下一行：
string blah = getBags() + "blah";
No errors.
没有错误。
What am I not understanding here?
我在这里不明白什么？
采纳答案by Ferruccio
"Check in "is actually a const char *.
Adding getBags()(an int) to it yields another const char*. The compiler error is generated because you cannot add two pointers.
"Check in "实际上是一个const char *. 将getBags()(an int)添加到它会产生另一个const char*。生成编译器错误是因为您无法添加两个指针。
You need to convert both "Check in "and getBags()to strings before concatenating them:
需要转换都"Check in "和getBags()以串联它们之前的字符串：
string bags = std::string("Check in ") + std::to_string(getBags()) + " bags";
" bags"will be implicitly converted to a string.
" bags"将被隐式转换为 a string。
回答by SHR
when using + to append strings the first element must have operator+, const char* doesn't have it.
当使用 + 附加字符串时，第一个元素必须有 operator+，而 const char* 没有。
therefore you should to make a string from it:
因此，您应该从中制作一个字符串：
string bags = string("Check in ") + getBags() + " bags";
or to do it in to steps:
或按以下步骤操作：
string bags = string("Check in ") + getBags() + " bags";
EDIT:More problem is the int returned from the method, for some reason, string doesn't have operator+ for int.
编辑：更多的问题是从方法返回的 int，出于某种原因，字符串没有用于 int 的 operator+。
So you better use stringstream like this:
所以你最好像这样使用 stringstream：
#include <sstream>
....
ostringstream s;
s<<"Check in " << getBags() << " bags";
string bags = s.str();
回答by Lee Thomas
Try std::to_string(getBags()). You can only concatenate strings with strings.
试试std::to_string(getBags())。您只能将字符串与字符串连接起来。
