Is there a way to check what value a type will be converted to before using it?

Question

Ryan McCombe · Accepted Answer

Yes! The easiest way to check how a value will be converted is to create a small test program. Let's look at how to do this safely:

1#include <iostream>
2using namespace std;
3
4int main() {
5  // Store the original value in a variable
6  double Original{3.7};
7
8  // Then create new variable of the target type
9  int Converted{static_cast<int>(Original)};
10
11  // Print both to see the difference
12  cout << "Original value: " << Original;
13  cout << "\nConverted value: " << Converted;
14}

1Original value: 3.7
2Converted value: 3

We can do the same thing with other types too. Here's how different number types convert to bool:

1#include <iostream>
2using namespace std;
3
4int main() {
5  int Zero{0};
6  int One{1};
7  int FortyTwo{42};
8
9  bool FromZero{static_cast<bool>(Zero)};
10  bool FromOne{static_cast<bool>(One)};
11  bool FromFortyTwo{static_cast<bool>(FortyTwo)};
12
13  // Makes cout show "true"/"false"
14  cout << std::boolalpha;
15  cout << "0 converts to: " << FromZero << "\n";
16  cout << "1 converts to: " << FromOne << "\n";
17  cout << "42 converts to: " << FromFortyTwo;
18}

10 converts to: false
21 converts to: true
342 converts to: true

Notice we used static_cast<>() in these examples. This is the safe way to do conversions in C++. While implicit conversions will work too, using static_cast<>() makes it clear to other programmers (and yourself!) that you intended to do the conversion.

Remember that some conversions might lose data (like when converting from double to int), so it's always good to test your conversions with realistic values before using them in your actual program.

Implicit Conversions and Narrowing Casts

Testing Type Conversions

Implicit Conversions and Narrowing Casts

Intro to C++ Programming

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