expected primary-expression before ']' token 这是什么错误

[guzhebo001]

G:\C++初级\string2.cpp In constructor `String::String()':
54 G:\C++初级\string2.cpp expected primary-expression before ']' token

以上就是错误的提示，知道的请告知下，谢谢

#include<iostream>
#include<string.h>
using namespace std;

class String
{
private:
char *ptr;
public:
String();
String(char *s);
String(const String &src);
~String();

String& operator = (const String &src)
{
cpy(src.ptr); return *this;
}

String& operator = (char *s)
{
cpy(s); return *this;
}

String operator + (char *s);

int operator == (const String &other);
operator char*() {return ptr;}

void cat(char *s);
void cpy(char *s);

};

int main()
{
String a,b,c;
a = "I";
b = "am";
c = "so";
String d = a + b + c + "very happpy!\n";
cout<<d;

return 0;
}


//---------------------------------------

//String 类的成员函数

String::String()
{
ptr = new char[];
ptr[0] = '\0';
}

String::String(char *s)
{
int n = strlen(s);
ptr = new char[n+1];
strcpy(ptr,s);
}


String::String(const String &src)
{
int n = strlen(src.ptr);
ptr = new char[n+1];
strcpy(ptr,src.ptr);
}

String::~String()
{
delete [] ptr;
}

int String::operator == (const String &other)
{
return (strcmp(ptr,other.ptr) == 0);
}

String String::operator + (char *s)
{
String new_str(ptr);
new_str.cat(s);
return new_str;
}

//cpy函数--用于拷贝字符串
//
void String::cpy(char *s)
{
delete [] ptr;
int n = strlen(s);
ptr = new char[n+1];
strcpy(ptr,s);
}

//cat函数--用于连接字符串
//
void String::cat(char *s)
{
//为新的字符串数据分配足够的空间
int n = strlen(ptr) + strlen(s);
char *p1 = new char[n+1];

//将数据拷贝到新的内存块
strcpy(p1,ptr);
strcat(p1,s);

//回收旧的内存快，并更新ptr
delete [] ptr;
ptr = p1;
}

[tangshuiling]

错误信息已经告诉你啦
String::String() 
  { 
    ptr = new char[]; //没有这种用法直接ptr=new char;
    ptr[0] = '\0'; 
  } 

[waizqfor]

code 没错啊 拿VC6试了一下 没出现LZ的错误

#include <iostream> 
#include <string.h> 
using namespace std; 

class String 
{ 
private: 
char *ptr; 
public: 
String(); 
String(char *s); 
String(const String &src); 
~String(); 

String& operator = (const String &src) 
{ 
  cpy(src.ptr); return *this; 
} 

String& operator = (char *s) 
{ 
  cpy(s); return *this; 
} 

String operator + (char *s); 

int operator == (const String &other); 
operator char*() {return ptr;} 

void cat(char *s); 
void cpy(char *s); 

}; 

  int main() 
  { 
    String a,b,c; 
    a = "I"; 
    b = "am"; 
    c = "so"; 
    String d = a + b + c + "very happpy!\n"; 
    cout <<d; 
    
  return 0; 
  } 


  //--------------------------------------- 

  //String 类的成员函数 

  String::String() 
  { 
    ptr = new char[]; 
    ptr[0] = '\0'; 
  } 

  String::String(char *s) 
  { 
    int n = strlen(s); 
    ptr = new char[n+1]; 
    strcpy(ptr,s); 
  } 


  String::String(const String &src) 
  { 
    int n = strlen(src.ptr); 
    ptr = new char[n+1]; 
    strcpy(ptr,src.ptr); 
  } 

  String::~String() 
  { 
    delete [] ptr; 
  } 

  int String::operator == (const String &other) 
  { 
    return (strcmp(ptr,other.ptr) == 0); 
  } 

  String String::operator + (char *s) 
  { 
    String new_str(ptr); 
    new_str.cat(s); 
    return new_str; 
  } 

  //cpy函数--用于拷贝字符串 
  // 
  void String::cpy(char *s) 
  { 
    delete [] ptr; 
    int n = strlen(s); 
    ptr = new char[n+1]; 
    strcpy(ptr,s); 
  } 

  //cat函数--用于连接字符串 
  // 
  void String::cat(char *s) 
  { 
    //为新的字符串数据分配足够的空间 
  int n = strlen(ptr) + strlen(s); 
  char *p1 = new char[n+1]; 

  //将数据拷贝到新的内存块 
  strcpy(p1,ptr); 
  strcat(p1,s); 

  //回收旧的内存快，并更新ptr 
  delete [] ptr; 
  ptr = p1; 
  }

[tangshuiling]

vc6很多地方还是不符合标准的，c++标准中new char[]中括号中必须有值

[guzhebo001]

再次感谢 waizqfor dev-C++ 真的很好用

ptr = new char[]; 而这里 改成ptr = new char[1]; 就好了，刚发现的，

输出是 Iamsovery happy!

[nineforever]

new char[]

->

new char[1]

[waizqfor]

[Quote=引用 4 楼 guzhebo001 的回复:]
再次感谢 waizqfor dev-C++ 真的很好用

ptr = new char[]; 而这里 改成ptr = new char[1]; 就好了，刚发现的，

输出是 Iamsovery happy!
[/Quote]
呵呵 我刚才忙的别的 没把你的程序用别的编译器 试 所以so 对不起了 改正就好了

[wenjianhn]

学习了