Error handling in ERC20 and ERC721

contracts/token/ERC20/ERC20.sol

@@ -125,7 +125,7 @@ contract ERC20 is IERC20 {
      * @param value The amount to be transferred.
      */
     function _transfer(address from, address to, uint256 value) internal {
-        require(to != address(0));
+        require(to != address(0), "transfer recipient is zero");
 
         _balances[from] = _balances[from].sub(value);
         _balances[to] = _balances[to].add(value);
@@ -140,7 +140,7 @@ contract ERC20 is IERC20 {
      * @param value The amount that will be created.
      */
     function _mint(address account, uint256 value) internal {
-        require(account != address(0));
+        require(account != address(0), "mint recipient is zero");
 
         _totalSupply = _totalSupply.add(value);
         _balances[account] = _balances[account].add(value);
@@ -154,7 +154,7 @@ contract ERC20 is IERC20 {
      * @param value The amount that will be burnt.
      */
     function _burn(address account, uint256 value) internal {
-        require(account != address(0));
+        require(account != address(0), "burn recipient is zero");
 
         _totalSupply = _totalSupply.sub(value);
         _balances[account] = _balances[account].sub(value);
@@ -168,8 +168,8 @@ contract ERC20 is IERC20 {
      * @param value The number of tokens that can be spent.
      */
     function _approve(address owner, address spender, uint256 value) internal {
-        require(spender != address(0));
-        require(owner != address(0));
+        require(spender != address(0), "spender address can't be zero");
+        require(owner != address(0), "owner address can't be zero");
 
         _allowed[owner][spender] = value;
         emit Approval(owner, spender, value);

contracts/token/ERC721/ERC721.sol

@@ -57,7 +57,7 @@ contract ERC721 is ERC165, IERC721 {
      * @return uint256 representing the amount owned by the passed address
      */
     function balanceOf(address owner) public view returns (uint256) {
-        require(owner != address(0));
+        require(owner != address(0), "balance query for zero address");
         return _ownedTokensCount[owner].current();
     }
 
@@ -68,7 +68,7 @@ contract ERC721 is ERC165, IERC721 {
      */
     function ownerOf(uint256 tokenId) public view returns (address) {
         address owner = _tokenOwner[tokenId];
-        require(owner != address(0));
+        require(owner != address(0), "owner doesn't exist");
         return owner;
     }
 
@@ -82,8 +82,8 @@ contract ERC721 is ERC165, IERC721 {
      */
     function approve(address to, uint256 tokenId) public {
         address owner = ownerOf(tokenId);
-        require(to != owner);
-        require(msg.sender == owner || isApprovedForAll(owner, msg.sender));
+        require(to != owner, "transfer recipient must not be current owner");
+        require(msg.sender == owner || isApprovedForAll(owner, msg.sender), "sender is not owner nor is approved");
 
         _tokenApprovals[tokenId] = to;
         emit Approval(owner, to, tokenId);
@@ -96,7 +96,7 @@ contract ERC721 is ERC165, IERC721 {
      * @return address currently approved for the given token ID
      */
     function getApproved(uint256 tokenId) public view returns (address) {
-        require(_exists(tokenId));
+        require(_exists(tokenId), "tokenId to aprove doesn't exists");
         return _tokenApprovals[tokenId];
     }
 
@@ -107,7 +107,7 @@ contract ERC721 is ERC165, IERC721 {
      * @param approved representing the status of the approval to be set
      */
     function setApprovalForAll(address to, bool approved) public {
-        require(to != msg.sender);
+        require(to != msg.sender, "sender can't be receipent");
         _operatorApprovals[msg.sender][to] = approved;
         emit ApprovalForAll(msg.sender, to, approved);
     }
@@ -131,7 +131,7 @@ contract ERC721 is ERC165, IERC721 {
      * @param tokenId uint256 ID of the token to be transferred
      */
     function transferFrom(address from, address to, uint256 tokenId) public {
-        require(_isApprovedOrOwner(msg.sender, tokenId));
+        require(_isApprovedOrOwner(msg.sender, tokenId), "sender is not owner nor is approved");
 
         _transferFrom(from, to, tokenId);
     }
@@ -186,6 +186,7 @@ contract ERC721 is ERC165, IERC721 {
      * is an operator of the owner, or is the owner of the token
      */
     function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
+        require(_exists(tokenId), "tokenId doesn't exist");
         address owner = ownerOf(tokenId);
         return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
     }
@@ -197,8 +198,8 @@ contract ERC721 is ERC165, IERC721 {
      * @param tokenId uint256 ID of the token to be minted
      */
     function _mint(address to, uint256 tokenId) internal {
-        require(to != address(0));
-        require(!_exists(tokenId));
+        require(to != address(0), "mint recipient is zero");
+        require(!_exists(tokenId), "tokenId to mint already exist");
 
         _tokenOwner[tokenId] = to;
         _ownedTokensCount[to].increment();
@@ -214,7 +215,8 @@ contract ERC721 is ERC165, IERC721 {
      * @param tokenId uint256 ID of the token being burned
      */
     function _burn(address owner, uint256 tokenId) internal {
-        require(ownerOf(tokenId) == owner);
+        require(_exists(tokenId), "tokenId to burn doesn't exist");
+        require(ownerOf(tokenId) == owner, "can only performed by owner");
 
         _clearApproval(tokenId);
 
@@ -241,8 +243,9 @@ contract ERC721 is ERC165, IERC721 {
      * @param tokenId uint256 ID of the token to be transferred
      */
     function _transferFrom(address from, address to, uint256 tokenId) internal {
-        require(ownerOf(tokenId) == from);
-        require(to != address(0));
+        require(_exists(tokenId), "tokenId to transfer doesn't exist");
+        require(ownerOf(tokenId) == from, "can only performed by owner");
+        require(to != address(0), "transfer recipient is zero");
 
         _clearApproval(tokenId);
 

contracts/token/ERC721/ERC721Burnable.sol

@@ -12,7 +12,7 @@ contract ERC721Burnable is ERC721 {
      * @param tokenId uint256 id of the ERC721 token to be burned.
      */
     function burn(uint256 tokenId) public {
-        require(_isApprovedOrOwner(msg.sender, tokenId));
+        require(_isApprovedOrOwner(msg.sender, tokenId), "sender is not owner nor is approved");
         _burn(tokenId);
     }
 }