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具有功能签名的Const关键字(Const keyword with function signature)

 这是我的简单代码：  
#include <iostream>

using namespace std;

class alloc { };

template <typename T, typename Alloc = alloc>
class vector
{
public:
    void swap(vector<T,Alloc> &v) { cout << "swap()" << endl; }
};


template <typename T, typename Alloc>
void swap(const vector<T,Alloc> &v1,const vector<T,Alloc> &v2)
{
    v1.swap(v2);
}

int main()
{
    vector<int> x;
    vector<int> y;

    swap(x,y);

    return 0;
}
 
 代码片段运行没有问题，但我无法获得任何输出  
 然后我删除const关键字。  
void swap(vector<T,Alloc> &v1,vector<T,Alloc> &v2)
 
 我得到输出swap()  
 我读过“原因是参数的const只适用于函数内部的本身，因为它正在处理数据副本，这意味着函数签名无论如何都是相同的。”  
 所以我认为写或不写const之间没有区别。如果我坚持在这里写const，我如何修改代码以获得输出swap() 

Here is my simple code: 
#include <iostream>

using namespace std;

class alloc { };

template <typename T, typename Alloc = alloc>
class vector
{
public:
    void swap(vector<T,Alloc> &v) { cout << "swap()" << endl; }
};


template <typename T, typename Alloc>
void swap(const vector<T,Alloc> &v1,const vector<T,Alloc> &v2)
{
    v1.swap(v2);
}

int main()
{
    vector<int> x;
    vector<int> y;

    swap(x,y);

    return 0;
}
 
The code snippet run without problem.But i can't get any outputs 
Then I delete const keyword. 
void swap(vector<T,Alloc> &v1,vector<T,Alloc> &v2)
 
I get output swap() 
I have read "The reason is that const for the parameter only applies locally within the function, since it is working on a copy of the data. This means the function signature is really the same anyways." 
So i thought there is no difference between write or not write const.if i insist to write const here,how do i modify the code to get the output swap()

原文：https://stackoverflow.com/questions/36808478

更新时间：2022-02-14 15:02

最满意答案

 我写了自定义方法  
- (CLLocationCoordinate2D) geoCodeUsingAddress:(NSString *)address
{

    double latitude = 0, longitude = 0;

    NSString *esc_addr =  [address  
                             stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];

    NSString *req = [NSString  

      stringWithFormat:@"http://maps.google.com/maps/api/geocode/json             
                                   sensor=false&address=%@", esc_addr];

    NSString *result = [NSString stringWithContentsOfURL:[NSURL URLWithString:req] 
                                         encoding:NSUTF8StringEncoding error:NULL];
    if (result) {

        NSScanner *scanner = [NSScanner scannerWithString:result];

        if ([scanner scanUpToString:@"\"lat\" :" intoString:nil] && 
            [scanner scanString:@"\"lat\" :" intoString:nil]) {

                   [scanner scanDouble:&latitude];

                   if ([scanner scanUpToString:@"\"lng\" :" intoString:nil] && 
                       [scanner scanString:@"\"lng\" :" intoString:nil]) {

                                 [scanner scanDouble:&longitude];
                   }
        }
    }
    CLLocationCoordinate2D center;
    center.latitude = latitude;
    center.longitude = longitude;

    return center;
}
 
 OUTPUT：  
 我什么时候通过字符串然后自动指向GoogleMap中的所需位置。 

I wrote the custom method 
- (CLLocationCoordinate2D) geoCodeUsingAddress:(NSString *)address
{

    double latitude = 0, longitude = 0;

    NSString *esc_addr =  [address  
                             stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];

    NSString *req = [NSString  

      stringWithFormat:@"http://maps.google.com/maps/api/geocode/json             
                                   sensor=false&address=%@", esc_addr];

    NSString *result = [NSString stringWithContentsOfURL:[NSURL URLWithString:req] 
                                         encoding:NSUTF8StringEncoding error:NULL];
    if (result) {

        NSScanner *scanner = [NSScanner scannerWithString:result];

        if ([scanner scanUpToString:@"\"lat\" :" intoString:nil] && 
            [scanner scanString:@"\"lat\" :" intoString:nil]) {

                   [scanner scanDouble:&latitude];

                   if ([scanner scanUpToString:@"\"lng\" :" intoString:nil] && 
                       [scanner scanString:@"\"lng\" :" intoString:nil]) {

                                 [scanner scanDouble:&longitude];
                   }
        }
    }
    CLLocationCoordinate2D center;
    center.latitude = latitude;
    center.longitude = longitude;

    return center;
}
 
OUTPUT: 
When ever I passed string then automatically pointing the required location in GoogleMap.

具有功能签名的Const关键字(Const keyword with function signature)

最满意答案

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