警告：有符号和无符号整数表达式之间的比较 [英] Warning: Comparison between signed and unsigned integer expression

问题描述

我正在codepad.org上运行以下代码，但出现此错误。 在成员函数'double Xchange :: getprice（std :: string）'中：
第87行：警告：有符号和无符号整数表达式之间的比较

I am running hhe following code on codepad.org and I am getting this error. "In member function 'double Xchange::getprice(std::string)': Line 87: warning: comparison between signed and unsigned integer expressions"

这是我的代码：

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class Xchange
{
public:
    Xchange();//does nothing (?)

    double getprice(string symbol);

private:
    vector <Stock> stocks;
};

double Xchange::getprice(string symbol)
{
    for(int i=0; i < stocks.size(); i++) {
        if(stocks[i].getsymbol()==symbol) {
            return stocks[i].getprice();
        }
    }

    return -1; //means not found
}

推荐答案

：

for(int i=0; i < stocks.size(); i++)

i 是一个有符号整数， stocks.size （）未签名。您可以使用 std :: size_t ，或者，如果要更加精确，请使用 vector< Stock> :: size_type

i is a signed integer, stocks.size() is unsigned. You can use std::size_t, or, if you want to be more precise, use the vector<Stock>::size_type.

for(vector<Stock>::size_type i=0; i < stocks.size(); i++) { .... }

此警告试图防止的问题是负号到无符号转换会产生大量的转换，很可能不是您想要的转换。除此之外，有符号整数的数值范围与相同大小的无符号整数的数值范围不同。

The problem this warning is trying to prevent is that a negative signed to unsigned conversion yields a large number and is most likely not what you want. Besides that, the numerical range of a signed integer is no the same as that of an unsigned one of the same size.

请参见C++类型以获取更多信息。

See C++ types for more information.

请注意，这在C ++ 11中更容易：

Note that this is easier in C++11:

for(const auto& stock : stocks)
{
    if(stock.getsymbol()==symbol) //added getsymbol "()"
    {
        return stock.getprice();
    }
}

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