Empty of Struct, String, Array or Mapping in Solidity
There is no concept of NULL in solidity. Empty of a struct, string, array or mapping in solidity is treated as zero-value of each type, e.g. zero of unit type is 0.
string
Normally, an empty string means its length is 0. But unfortunately, string in solidity has no attribute or function to retrieve its length. Two following ways can be used to accomplish this:
string x;
if(bytes(x).length == 0) {……}
// is empty
or
string x;
if (sha3(x) != sha3(“”)) {……}
// is not empty
struct
In solidity, a struct does not need to be new’ed for its members to be accessible, where all the members are empty with their members having zero-value of each type.
struct Transaction {
address to;
unit256 value;
bytes data;
}
Transaction private tx;
Members of tx can be accessed even tx has been assigned, tx.to has its zero-value: 0.
array
An empty array is an array with length of 0.
address[] memory addrList;
if (addrList.length == 0) {……}
// is empty
mapping
Array and mapping are accessible immediately after declaration.
mapping(string =>Transaction ) txMapping;
…
if （txMapping[“xx”].value == 0） ｛……｝
Although the Transaction struct object txMapping[“xx”] has not been assigned, the ‘value’ member of this ‘virtual’ object can be accessed directly.
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