成员引用基本类型“ double”不是结构或联合

Question

I'm new to C++ and can't figure out how to fix the error, would greatly appreciate for your help The part where the error occurs I'm trying to input the radius into the cirArea[] array, but it does not seem to work.

Here's a part of my code:

int main(){
    Circle *area;
    double cirRadius;
    int numCircle;

    cout << "How many circles?" << endl;
    cin >> numCircle;
    double cirArea[numCircle];

    for (int i = 0; i < numCircle; i++){
        cout << "Enter the radius: ";
        cin >> cirRadius;
        cirArea[i].setRadius(cirRadius);
    }
}

For setRadius():

void Circle::setRadius(double r){
    if (r >= 0)
        radius = r;
    else {
        cout << "Invalid radius\n";
        exit(EXIT_FAILURE);
    }
}

so here's the ERROR:

member reference base type 'double' is not a structure or union
            cirArea[i].setRadius(cirRadius);
            ~~~~~~~~~~^~~~~~~~~~

Answer 1

You cannot create an statically allocated array with 'not a constant size'. Second issue is that cirArea is not of Circle type but of double .

You would have to allocate it dynamically (and delete it afterwards) and make it Circle type:

int main(){
    double cirRadius;
    int numCircle;

    cout << "How many circles?" << endl;
    cin >> numCircle;
    Circle *area = new Circle[numCircle];

    for (int i = 0; i < numCircle; i++){
        cout << "Enter the radius: ";
        cin >> cirRadius;
        cirArea[i].setRadius(cirRadius);
    }

    delete[] area;
}

But a preferable way of doing it would be to use a std::vector

#include <iostream>
#include <cstdlib>
#include <vector>

struct Circle
{
    double radius;

    void setRadius(double r){
        if (r >= 0)
            radius = r;
        else {
            std::cout << "Invalid radius\n";
            exit(EXIT_FAILURE);
        }
    }    
}   ; 

int main(){
    double cirRadius;
    int numCircle;

    std::cout << "How many circles?\n";
    std::cin >> numCircle;
    std::vector<Circle> area;
    area.reserve(numCircle);

    for (int i = 0; i < numCircle; ++i){
        std::cout << "Enter the radius: ";
        std::cin >> cirRadius;
        area.emplace_back();
        area.back().setRadius(cirRadius);
    }

    for( Circle& i : area )
    {
      std::cout << i.radius << '\n';
    }
}

http://cpp.sh/3l7ti

Answer 2

Your cirArea has a type double, not Circle , so you cant't apply on it Circle method. Change the type of cirArea from double to Circule to fix the error.

Answer 3

It's essentially this statement that's at fault:

double cirArea[numCircle];

Variable length arrays are not supported in C++, and I think you mean to use a Circle object as the element rather than a double .

Use a std::vector<Circle> instead to model a vectors of Circle objects.

Answer 4

The problem is that cirArea is an array of double and not of Circle . The double type does of course not have Circle member functions (or any member functions at all).

Since your compiler seems to support variable-lenght arrays I suppose you really want

Circle cirArea[numCircle];

---

Variable-length arrays are not a standard C++ feature, if you want to make the program portable you should use std::vector like

std::vector<Circle> cirArea(numCircle);

Answer 5

double cirArea[numCircle]; is an array of double . It should be an array of Circle . However, numCircle is a non-const so you can not do that (even if the compiler allow it. It is not standared). You should use dynamically allocated array or even better a std::vector .

A complete C++ solution would be :

int main(){
    Circle *area;
    double cirRadius;
    int numCircle;

    cout << "How many circles?" << endl;
    cin >> numCircle;
    std::vector<Circle> cirArea;
    cirArea.reserve(numCircle);

    for (int i = 0; i < numCircle; i++){
        cout << "Enter the radius: ";
        cin >> cirRadius;
        cirArea.emplace_back(); 
        cirArea.back().setRadius(cirRadius);
    }
}

if Circle accept Radius as constructor argument , you may replace those two lines:

cirArea.emplace_back(); 
cirArea.back().setRadius(cirRadius);

with:

cirArea.emplace_back(cirRadius);