why implicit conversion is harmful in C++

Question

I understand that the keyword explicit can be used to prevent implicit conversion.

For example

Foo {

 public:
 explicit Foo(int i) {}
}
         


        

Answer 1

A real world example:

class VersionNumber
{
public:
    VersionNumber(int major, int minor, int patch = 0, char letter = '\0') : mMajor(major), mMinor(minor), mPatch(patch), mLetter(letter) {}
    explicit VersionNumber(uint32 encoded_version) { memcpy(&mLetter, &encoded_version, 4); }
    uint32 Encode() const { int ret; memcpy(&ret, &mLetter, 4); return ret; }

protected:
    char mLetter;
    uint8 mPatch;
    uint8 mMinor;
    uint8 mMajor;
};

VersionNumber v = 10; would almost certainly be an error, so the explicit keyword requires the programmer to type VersionNumber v(10); and - if he or she is using a decent IDE - they will notice through the IntelliSense popup that it wants an encoded_version.