I wrote a program which involves use of switch statements... However on compilation it shows:
Error: Jump to case label.
Why does it do that?
#include <iostream>
#include <cstdlib>
#include <fstream>
#include <string>
using namespace std;
class contact
{
public:
string name;
int phonenumber;
string address;
contact() {
name= "Noname";
phonenumber= 0;
address= "Noaddress";
}
};
int main() {
contact *d;
d = new contact[200];
string name,add;
int choice,modchoice,t;//Variable for switch statement
int phno,phno1;
int i=0;
int initsize=0, i1=0;//i is declared as a static int variable
bool flag=false,flag_no_blank=false;
//TAKE DATA FROM FILES.....
//We create 3 files names, phone numbers, Address and then abstract the data from these files first!
fstream f1;
fstream f2;
fstream f3;
string file_input_name;
string file_input_address;
int file_input_number;
f1.open("./names");
while(f1>>file_input_name){
d[i].name=file_input_name;
i++;
}
initsize=i;
f2.open("./numbers");
while(f2>>file_input_number){
d[i1].phonenumber=file_input_number;
i1++;
}
i1=0;
f3.open("./address");
while(f3>>file_input_address){
d[i1].address=file_input_address;
i1++;
}
cout<<"\tWelcome to the phone Directory\n";//Welcome Message
do{
//do-While Loop Starts
cout<<"Select :\n1.Add New Contact\n2.Update Existing Contact\n3.Display All Contacts\n4.Search for a Contact\n5.Delete a Contact\n6.Exit PhoneBook\n\n\n";//Display all options
cin>>choice;//Input Choice from user
switch(choice){//Switch Loop Starts
case 1:
i++;//increment i so that values are now taken from the program and stored as different variables
i1++;
do{
cout<<"\nEnter The Name\n";
cin>>name;
if(name==" "){cout<<"Blank Entries are not allowed";
flag_no_blank=true;
}
}while(flag_no_blank==true);
flag_no_blank=false;
d[i].name=name;
cout<<"\nEnter the Phone Number\n";
cin>>phno;
d[i1].phonenumber=phno;
cout<<"\nEnter the address\n";
cin>>add;
d[i1].address=add;
i1++;
i++;
break;//Exit Case 1 to the main menu
case 2:
cout<<"\nEnter the name\n";//Here it is assumed that no two contacts can have same contact number or address but may have the same name.
cin>>name;
int k=0,val;
cout<<"\n\nSearching.........\n\n";
for(int j=0;j<=i;j++){
if(d[j].name==name){
k++;
cout<<k<<".\t"<<d[j].name<<"\t"<<d[j].phonenumber<<"\t"<<d[j].address<<"\n\n";
val=j;
}
}
char ch;
cout<<"\nTotal of "<<k<<" Entries were found....Do you wish to edit?\n";
string staticname;
staticname=d[val].name;
cin>>ch;
if(ch=='y'|| ch=='Y'){
cout<<"Which entry do you wish to modify ?(enter the old telephone number)\n";
cin>>phno;
for(int j=0;j<=i;j++){
if(d[j].phonenumber==phno && staticname==d[j].name){
cout<<"Do you wish to change the name?\n";
cin>>ch;
if(ch=='y'||ch=='Y'){
cout<<"Enter new name\n";
cin>>name;
d[j].name=name;
}
cout<<"Do you wish to change the number?\n";
cin>>ch;
if(ch=='y'||ch=='Y'){
cout<<"Enter the new number\n";
cin>>phno1;
d[j].phonenumber=phno1;
}
cout<<"Do you wish to change the address?\n";
cin>>ch;
if(ch=='y'||ch=='Y'){
cout<<"Enter the new address\n";
cin>>add;
d[j].address=add;
}
}
}
}
break;
case 3 : {
cout<<"\n\tContents of PhoneBook:\n\n\tNames\tPhone-Numbers\tAddresses";
for(int t=0;t<=i;t++){
cout<<t+1<<".\t"<<d[t].name<<"\t"<<d[t].phonenumber<<"\t"<<d[t].address;
}
break;
}
}
}
while(flag==false);
return 0;
}
This question is related to
c++
C++11 standard on jumping over some initializations
JohannesD gave an explanation, now for the standards.
The C++11 N3337 standard draft 6.7 "Declaration statement" says:
3 It is possible to transfer into a block, but not in a way that bypasses declarations with initialization. A program that jumps (87) from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type, class type with a trivial default constructor and a trivial destructor, a cv-qualified version of one of these types, or an array of one of the preceding types and is declared without an initializer (8.5).
87) The transfer from the condition of a switch statement to a case label is considered a jump in this respect.
[ Example:
void f() { // ... goto lx; // ill-formed: jump into scope of a // ... ly: X a = 1; // ... lx: goto ly; // OK, jump implies destructor // call for a followed by construction // again immediately following label ly }— end example ]
As of GCC 5.2, the error message now says:
crosses initialization of
C
C allows it: c99 goto past initialization
The C99 N1256 standard draft Annex I "Common warnings" says:
2 A block with initialization of an object that has automatic storage duration is jumped into
JohannesD's answer is correct, but I feel it isn't entirely clear on an aspect of the problem.
The example he gives declares and initializes the variable i in case 1, and then tries to use it in case 2. His argument is that if the switch went straight to case 2, i would be used without being initialized, and this is why there's a compilation error. At this point, one could think that there would be no problem if variables declared in a case were never used in other cases. For example:
switch(choice) {
case 1:
int i = 10; // i is never used outside of this case
printf("i = %d\n", i);
break;
case 2:
int j = 20; // j is never used outside of this case
printf("j = %d\n", j);
break;
}
One could expect this program to compile, since both i and j are used only inside the cases that declare them. Unfortunately, in C++ it doesn't compile: as Ciro Santilli ???? ???? ??? explained, we simply can't jump to case 2:, because this would skip the declaration with initialization of i, and even though case 2 doesn't use i at all, this is still forbidden in C++.
Interestingly, with some adjustments (an #ifdef to #include the appropriate header, and a semicolon after the labels, because labels can only be followed by statements, and declarations do not count as statements in C), this program does compile as C:
// Disable warning issued by MSVC about scanf being deprecated
#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#endif
#ifdef __cplusplus
#include <cstdio>
#else
#include <stdio.h>
#endif
int main() {
int choice;
printf("Please enter 1 or 2: ");
scanf("%d", &choice);
switch(choice) {
case 1:
;
int i = 10; // i is never used outside of this case
printf("i = %d\n", i);
break;
case 2:
;
int j = 20; // j is never used outside of this case
printf("j = %d\n", j);
break;
}
}
Thanks to an online compiler like http://rextester.com you can quickly try to compile it either as C or C++, using MSVC, GCC or Clang. As C it always works (just remember to set STDIN!), as C++ no compiler accepts it.
Declaration of new variables in case statements is what causing problems. Enclosing all case statements in {} will limit the scope of newly declared variables to the currently executing case which solves the problem.
switch(choice)
{
case 1: {
// .......
}break;
case 2: {
// .......
}break;
case 3: {
// .......
}break;
}
Source: Stackoverflow.com