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Conversion Operators and Casts
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As you know, in classic C++ you can define type conversion operators to enable automatic conversions between your type and another type. You can do this in managed types as well as in C++/CLI. The additional option you have in C++/CLI is to specify whether the conversion requires an explicit cast, or not. You do this with the explicit keyword. While the explicit keyword is also used in classic C++, in classic C++ it is used only on constructors, to prevent the constructor from being used to define an implicit conversion. In C++/CLI, the situation is different. Constructors for managed types are never used for implicit conversions, whether or not the explicit keyword is used on them, so using the keyword would be redundant. However, the keyword is used on conversion operators. Without the keyword, the conversion operator is assumed to be implicit, as it is in classic C++. With the keyword, the conversion operator is only invoked with an explicit cast (see Listing 7-23). Listing 7-23. Using explicit with a Conversion Operator // explicit_conversion.cpp using namespace System; value class BigIntExplicit { __int64 m_i;
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public: explicit BigIntExplicit(int i) : m_i(i) { } explicit operator int() { return m_i; } explicit static operator BigIntExplicit(int i) { return BigIntExplicit(i); } void takeBigIntExplicit(BigIntExplicit b) {} }; value class BigIntImplicit { __int64 m_i; public: BigIntImplicit(int i) : m_i(i) { } operator int() { return m_i; } static operator BigIntImplicit(int i) { return BigIntImplicit(i); } void takeBigIntImplicit(BigIntImplicit b) {} }; int main() { BigIntExplicit b_exp(400); BigIntImplicit b_imp(500); int i = safe_cast<int>( b_exp ); // OK: requires explicit cast int j = b_imp; // OK: implicit
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// int cannot implicitly be converted to BigInt1 and BigInt2 // with the constructor; instead, you define the static conversion operator. // This is different from standard C++, which uses the constructor // for such implicit conversions. b_exp.takeBigIntExplicit(safe_cast<BigIntExplicit>(i)); b_imp.takeBigIntImplicit(j); }
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In Listing 7-23, observe several things: The explicit keyword on operator int used in BigIntExplicit disallows implicit conversions from BigIntExplicit to int. This is useful if you want to prevent unexpected conversions from taking place without an explicit cast. The explicit keyword on the constructor for BigIntExplicit has no effect. We need the static operator BigInt that takes an int to convert. Again we have the choice of making the operator explicit or implicit. We use the preferred safe_cast casting mechanism. C-style casts (i.e., using the target type name in parentheses) are supported in C++/CLI, and if used, evaluate to another type of cast. All of the preceding conversion functions will work with C-style casts and static_cast as well. In the next sections, I ll explain casting in C++/CLI and review the various casting mechanisms in classic C++, which are also available in C++/CLI. The next chapter will revisit dynamic_cast in the context of inheritance.
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C-Style Casts
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C-style casts are casts that use the parentheses syntax used in C. The use of C-style casts is not recommended since it can be difficult to tell what type of cast will actually be performed. However, there are definite rules that determine the type of cast actually performed. Basically, the compiler attempts various types of casts ranging from the safest to the least safe. If possible, the validity of the cast will be determined at compile time. If the types involved in the conversion differ only by whether or not they are constant, the cast is interpreted as a const_cast. If the two types are in the same inheritance chain or are both interface handles, the safe_cast is used. Next, the compiler will attempt to interpret the cast as a static_cast. This will succeed if the types have a defined conversion (for example, a conversion operator). Also, if a safe_cast or static_cast will work if combined with a const_cast, the compiler will do so. Finally, if the compiler cannot determine whether a static_cast is valid, a runtime check will be performed. If the runtime check fails, an InvalidCastException will be generated.
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