This lesson continues our exploration of switch statements that we started in the prior lesson 7.4 -- Switch statement basics. In the prior lesson, we mentioned that each set of statements underneath a label should end in a break statement or a return statement.
In this lesson, we’ll explore why, and talk about some switch scoping issues that sometimes trip up new programmers.
Fallthrough
When a switch expression matches a case label or optional default label, execution begins at the first statement following the matching label. Execution will then continue sequentially until one of the following termination conditions happens:
- The end of the switch block is reached.
- Another control flow statement (typically a
breakorreturn) causes the switch block or function to exit. - Something else interrupts the normal flow of the program (e.g. the OS shuts the program down, the universe implodes, etc…)
Note that the presence of another case label is not one of these terminating conditions -- thus, without a break or return, execution will overflow into subsequent cases.
Here is a program that exhibits this behavior:
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#include <iostream> int main() { switch (2) { case 1: // Does not match std::cout << 1 << '\n'; // Skipped case 2: // Match! std::cout << 2 << '\n'; // Execution begins here case 3: std::cout << 3 << '\n'; // This is also executed case 4: std::cout << 4 << '\n'; // This is also executed default: std::cout << 5 << '\n'; // This is also executed } return 0; } |
This program outputs the following:
2 3 4 5
This is probably not what we wanted! When execution flows from a statement underneath a label into statements underneath a subsequent label, this is called fallthrough.
Warning
Once the statements underneath a case or default label have started executing, they will overflow (fallthrough) into subsequent cases. Break or return statements are typically used to prevent this.
Since fallthrough is rarely desired or intentional, many compilers and code analysis tools will flag fallthrough as a warning.
The [[fallthrough]] attribute
Commenting intentional fallthrough is a common convention to tell other developers that fallthrough is intended. While this works for other developers, the compiler and code analysis tools don’t know how to interpret comments, so it won’t get rid of the warnings.
To help address this, C++17 adds a new attribute called [[fallthrough]] that can be used in conjunction with a null statement to indicate that fallthough is intentional (and no warnings should be triggered):
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#include <iostream> int main() { switch (2) { case 1: std::cout << 1 << '\n'; break; case 2: std::cout << 2 << '\n'; // Execution begins here [[fallthrough]]; // intentional fallthrough -- note the semicolon to indicate the null statement case 3: std::cout << 3 << '\n'; // This is also executed break; } return 0; } |
This program prints:
2 3
And it should not generate any warnings about the fallthrough.
Best practice
Use the [[fallthrough]] attribute (along with a null statement) to indicate intentional fallthrough.
Sequential case labels
With if statements, you can use the logical OR operator to combine multiple tests into a single statement:
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bool isVowel(char c) { return (c=='a' || c=='e' || c=='i' || c=='o' || c=='u' || c=='A' || c=='E' || c=='I' || c=='O' || c=='U'); } |
This suffers from the same challenges that we presented in the introduction to switch statements: c gets evaluated multiple times and the reader has to make sure it is c that is being evaluated each time.
You can do something similar using switch statements by placing multiple case labels in sequence:
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bool isVowel(char c) { switch (c) { case 'a': // if c is 'a' case 'e': // or if c is 'e' case 'i': // or if c is 'i' case 'o': // or if c is 'o' case 'u': // or if c is 'u' case 'A': // or if c is 'A' case 'E': // or if c is 'E' case 'I': // or if c is 'I' case 'O': // or if c is 'O' case 'U': // or if c is 'U' return true; default: return false; } } |
Remember, execution begins at the first statement after a matching case label. Case labels aren’t statements (they’re labels), so they don’t count.
The first statement after all of the case statements in the above program is return true, so if any case labels match, the function will return true.
Thus, we can “stack” case labels to make all of those case labels share the same set of statements afterward. This is not considered fallthrough behavior, so use of comments or [[fallthrough]] is not needed here.
Switch case scoping
With if statements, you can only have a single statement after the if-condition, and that statement is considered to be implicitly inside a block:
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if (x > 10) std::cout << x << " is greater than 10\n"; // this line implicitly considered to be inside a block |
However, with switch statements, the statements after labels are all scoped to the the switch block. No implicit blocks are created.
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switch (1) { case 1: foo(); break; default: std::cout << "default case\n"; break; } |
In the above example, the 2 statements between the case 1 and the default label are scoped as part of the switch block, not a block implicit to case 1.
Variable declaration and initialization inside case statements
You can declare (but not initialize) variables inside the switch, both before and after the case labels:
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switch (1) { int a; // okay: declaration is allowed before the case labels int b{ 5 }; // illegal: initialization is not allowed before the case labels case 1: int y; // okay but bad practice: declaration is allowed within a case y = 4; // okay: assignment is allowed break; case 2: y = 5; // okay: y was declared above, so we can use it here too break; case 3: int z{ 4 }; // illegal: initialization is not allowed within a case break; } |
Note that although variable y was defined in case 1, it was used in case 2 as well. Because the statements under each case are not inside an implicit block, that means all statements inside the switch are part of the same scope. Thus, a variable defined in one case can be used in a later case, even if the case in which the variable is defined is never executed! Put another way, defining a variable without an initializer is just telling the compiler that the variable is now in scope from that point on. This doesn’t require the definition to actually be executed.
However, initialization of variables is disallowed and will cause a compile error. This is because initializing a variable does require execution, and initialization could be skipped over depending on which cases are executed.
If a case needs to define and/or initialize a new variable, best practice is to do so inside a block underneath the case statement:
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switch (1) { case 1: { // note addition of block here int x{ 4 }; // okay, variables can be initialized inside a block inside a case std::cout << x; break; } default: std::cout << "default case\n"; break; } |
Rule
If defining variables used in a case statement, do so in a block inside the case (or before the switch if appropriate)
Quiz time
Question #1
Write a function called calculate() that takes two integers and a char representing one of the following mathematical operations: +, -, *, /, or % (modulus). Use a switch statement to perform the appropriate mathematical operation on the integers, and return the result. If an invalid operator is passed into the function, the function should print an error. For the division operator, do an integer division.
Hint: “operator” is a keyword, variables can’t be named “operator”.
 7.6 -- Goto statements |
 Index |
 7.4 -- Switch statement basics |
hello
please you said "illegal: initialization is not allowed within a case" but when i try it in my ide it work fine,what is the reason for that?
do you mean initialization is allow but not prefer because we might not reach that case?
thank you for the great tutoriol
sorry i just figure it out, in default is allow in cases not allow
thank you very much!
What to do if the user input a CHAR instead of something numeric ?
what is the best way to go about solving this issue ?
A `char` is numeric.
Hi I have a question about the return value of the default case. In your solution for example you return 0, but that means that if the user inputs an invalid operator, lets say calculate(1, 2, 'k') for example, then the program will print:
1 k 2 is calculate(): Unhandled case
0
But I'd like the program to only print: calculate(): Unhandled case
How would you do that?
I though of checking the returned value with a conditional statement and using that to see if I should print it or not, but that wouldn't work because any value I return in the default case could also be an actual value returned from one of the valid cases.
I could choose a random number with little odds of ever being the result of the user's input, but that can't be the correct way to go about it...
`calculate` could throw an exception or return a type that can indicate errors. We haven't covered either yet.
`std::optional` isn't covered on learncpp I don't think
Hi, I'm not sure if it's a good place to leave this comment, I'm doing so because there are few comments and I hope admins can see it.
I have a question:
I started to learn cpp for competitive programming purposes and I was wondering if all of that knowledge from this tutorial is required for competitive programming, or I can just study some specific chapters/lessons on this site and then focus on algorithms and data structures? If not all of that knowledge is required, then which parts are? I've already covered lessons up to this chapter. I am looking forward to your reply.
Hi John!
I've never done competitive programming, so take this with a grain of salt.
If you're going to write real programs, or games, you'll need everything that is taught on learncpp and more.
If you're just going to write algorithms to solve very specific problems, you can probably do that after Chapter 10. Knowing more won't hurt. If you're going to stop after Chapter 10, and you're only going to do competitive programming, I don't think you had the need to learn C++ in the first place. You'll only know the features that all C-like languages have in common, meaning you could have done with an easier language like python or js/ts.
Okay, thank you! I've chosen c++ because this language and your tutorials were recommended on IOI site and I liked these lessons. You're doing great work guys :).
Great lesson! Two questions:
1. If I put more likely cases before less likely cases, could that improve performance?
2. I wanted to use doubles instead of integers. How does this look?
The order of switch cases shouldn't matter for performance. Best practice is to list the cases in alpha/numeric order, or whatever order makes for the easiest comprehension.
For your program:
* Initialize your variables
* Operator % doesn't make sense on doubles, you should remove this case.
Other than that, seems substantially similar to the reference solution.
I think you should add some tags beside the lesson's names in the Index page so that old visiters like me can find out the lessons you updated. And thanx so so much for this really helpful site!!!
You missed a ]
Thank you :)