英文:
Studying initialization in C++: what does "error: expected '(' for function-style cast or type construction" means in this case?
问题
我正在学习C++中的初始化。在这个概念验证中。我不理解编译器的建议。
(我已经知道在cin内声明变量不是一种容易和自然的方式（甚至可能不合法或不合适）。在这之前在外部声明它将更容易，也没有问题。但我提出这个问题只是为了更深入地理解发生了什么。试图更好地理解C++，尤其是初始化，这被认为不是一个简单的主题)。
#include <iostream>
int main()
{
std::cin >> int input_value;
return 0;
}
尝试编译时：
% g++ -Wall -o p44e1 p44e1.cc
p44e1.cc:5:18: error: expected '(' for function-style cast or type construction
std::cin >> int input_value;
~~~ ^
1 error generated.
英文:
I am studying initialization in C++. In this proof of concept. I don't understand compiler's advice.
(I already know that declaring a variable inside a cin is not the easy and natural (not even perhaps legal or even apropiate) way. Would be easy to just declare it outside before and no problem. But i asked the question just to understand more deeply what is going on. Trying to understand C++ in a better way and initialization in particular, which is known not to be a trivial subject).
#include <iostream>
int main ()
{
std::cin >> int input_value;
return 0;
}
when trying to compile:
% g++ -Wall -o p44e1 p44e1.cc
p44e1.cc:5:18: error: expected '(' for function-style cast or type construction
std::cin >> int input_value;
~~~ ^
1 error generated.
答案1
得分: 6
一个函数体由零个或多个语句组成。
有几种不同的语句。在这里，我们只关心两种：
-
声明语句，例如
int input_value; -
表达式语句。也就是
e;，其中e是一个表达式，意思是"用运算符连接的操作数，或者一个单独的操作数"。std::cin >> input_value;就是一个表达式语句，其中std::cin和input_value是操作数，>>是运算符。
因此，std::cin >> int input_value; 是明显无效的。int input_value; 必须是一个独立的语句，但你试图将它嵌入到另一个（表达式）语句中。
英文:
A function body consists of zero or more statements.
There are several kinds of statements. We only care about two here:
-
A declaration statement, e.g.
int input_value; -
An expression statement. That is,
e;. Whereeis an expression, meaning "operands connected with operators, or an individual operand".std::cin >> input_value;would be an expression statement, wherestd::cin >> input_valueis an expression (std::cinandinput_valueare operands, and>>is an operator).
So std::cin >> int input_value; is outright invalid. int input_value; must be a separate statement, but you tried to embed it into an other (expression) statement.
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