C ++中'：'标记之前的预期主表达式

Question

I keep getting these errors when trying to make this class and im not really sure what it means. I thought this was the correct way to do this but im not sure because im still new to c++.

expected primary-expression before ‘:’ token
expected ‘;’ before ‘:’ token    

Here is the header file:

#ifndef LEAKY_STACK_A_H
#define LEAKY_STACK_A_H
#include <string>
#include "LeakyStack.h"
using std::string;

class LeakyStackA : public LeakyStack {

public:
    /**
     * Constructor with specified max capacity
     *  \param the maximum capacity (default: 10)
     */
    LeakyStackA(int cap=DEF_CAPACITY);

    /**
     * Return the number of objects in the stack.
     * \return number of elements
     */
    int size() const;

    /**
     * Determine if the stack is currently empty.
     * \return true if empty, false otherwise.
     */
    bool empty() const;

    /**
     * Return a const reference to the top object in the stack.
     * \return const reference to top element
     * \throw runtime_error if the stack is empty
     */
    const std::string& top() const;

    /**
     * Insert an object at the top of the stack. If the stack
     * is already at capacity, the oldest element will be lost.
     * \param the new element
     */
    void push(const std::string& e);

    /**
     *  Remove the top object from the stack.
     *  \throw runtime_error if the stack is empty.
     */
    void pop();

private:
    enum { DEF_CAPACITY = 10 };                     // default stack capacity


    string* S;
    int capacity;
    int t;
    int n;
    int k;

};

#endif

And here is the .cpp file:

#include <stdexcept>
#include <iostream>
#include "LeakyStack.h"
#include "LeakyStackA.h"
using namespace std;

/**
 * Constructor with specified max capacity
 *  \param the maximum capacity (default: 10)
 */
LeakyStackA::LeakyStackA (int cap) {
   : S(new string[cap]), capacity(cap), t(-1); 

}
/**
 * Return the number of objects in the stack.
 * \return number of elements
 */
int LeakyStackA::size() const {
    return (t+1);    
}


/**
 * Determine if the stack is currently empty.
 * \return true if empty, false otherwise.
 */
bool LeakyStackA::empty() const {
    return (t < 0);  
}


/**
 * Return a const reference to the top object in the stack.
 * \return const reference to top element
 * \throw runtime_error if the stack is empty
 */
const string& LeakyStackA::top() const {
    if (empty()) throw runtime_error("Stack is Empty");
    return S[t];
}


/**
 * Insert an object at the top of the stack. If the stack
 * is already at capacity, the oldest element will be lost.
 * \param the new element
 */
void LeakyStackA::push(const string& e) {
    if (size() == capacity) {
    S[t--];
    S[t++] = e;
    }
    else {
      S[t++] = e;
    }
    //if (size() == capacity) throw runtime_error("Stack is Full");
    //S[++t] = e;
}

/**
 *  Remove the top object from the stack.
 *  \throw runtime_error if the stack is empty.
 */
void LeakyStackA::pop() {
    if(empty()) throw runtime_error("Stack is Empty");
    --t;

}

any help would be appreciated thanks.

Answer 1

The initializer list is part of the constructor definition, and written like this:

LeakyStackA::LeakyStackA (int cap)
: S(new string[cap]), capacity(cap), t(-1)
{  }

Answer 2

Here is the problem

LeakyStackA::LeakyStackA (int cap) {
   : S(new string[cap]), capacity(cap), t(-1); 

}

it should be like this:

LeakyStackA::LeakyStackA (int cap) 
   : S(new string[cap]), capacity(cap), t(-1) {}

Answer 3

This is not a direct answer to the question but to the general compiler error "expected primary-expression before ':' token"

In my case, I had typed std:string instead of std::string .

I share this here because it is the first occurrence of this specific error I found on StackOverflow and other people might have the same problem.