add capacity function to query the underlying allocated space

base/array.jl

@@ -1023,6 +1023,17 @@ function sizehint!(a::Vector, sz::Integer)
     a
 end
 
+"""
+    capacity(s)
+
+Returns the allocated buffer capacity of s.
+"""
+function capacity end
+
+function capacity(a::Vector)
+    return ccall(:jl_array_capacity, Csize_t, (Any,), a)
+end
+
 """
     pop!(collection) -> item
 

base/exports.jl

@@ -477,6 +477,7 @@ export
     any!,
     any,
     firstindex,
+    capacity,
     collect,
     count,
     delete!,

src/array.c

@@ -1116,6 +1116,11 @@ JL_DLLEXPORT void jl_array_sizehint(jl_array_t *a, size_t sz)
     }
 }
 
+JL_DLLEXPORT size_t jl_array_capacity(jl_array_t *a)
+{
+    return a->maxsize;
+}
+
 JL_DLLEXPORT jl_array_t *jl_array_copy(jl_array_t *ary)
 {
     size_t elsz = ary->elsize;