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Aggregates
It’s possible to use const
for aggregates, but you’re
virtually assured that the compiler will not be sophisticated enough to keep an
aggregate in its symbol table, so storage will be allocated. In these
situations, const means “a piece of storage that cannot be
changed.” However, the value cannot be used at compile time because the
compiler is not required to know the contents of the storage at compile time. In
the following code, you can see the statements that are
illegal:
//: C08:Constag.cpp
// Constants and aggregates
const int i[] = { 1, 2, 3, 4 };
//! float f[i[3]]; // Illegal
struct S { int i, j; };
const S s[] = { { 1, 2 }, { 3, 4 } };
//! double d[s[1].j]; // Illegal
int main() {} ///:~
In an
array definition, the compiler
must be able to generate code that moves the stack pointer to accommodate the
array. In both of the illegal definitions above, the compiler complains because
it cannot find a constant expression in the array
definition.
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